Revert "[clang] Remove #undef alloca workaround" (llvm#81649)

Reverts llvm#81534

llvm#81534 breaks building (Fuchsia) Clang toolchain on
Windows.

Log:
https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8756186536543250705/+/u/clang/install/stdout
Builder:
https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci/clang-windows-x64/b8756186536543250705/overview

```
FAILED: tools/clang/tools/extra/clang-include-fixer/tool/CMakeFiles/clang-include-fixer.dir/ClangIncludeFixer.cpp.obj 
C:\b\s\w\ir\x\w\cipd\bin\clang-cl.exe  /nologo -TP -DCLANG_REPOSITORY_STRING=\"https://llvm.googlesource.com/llvm-project\" -DGTEST_HAS_RTTI=0 -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_GLIBCXX_ASSERTIONS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -IC:\b\s\w\ir\x\w\llvm_build\tools\clang\tools\extra\clang-include-fixer\tool -IC:\b\s\w\ir\x\w\llvm-llvm-project\clang-tools-extra\clang-include-fixer\tool -IC:\b\s\w\ir\x\w\llvm-llvm-project\clang\include -IC:\b\s\w\ir\x\w\llvm_build\tools\clang\include -IC:\b\s\w\ir\x\w\recipe_cleanup\tensorflow-venv\store\python_venv-q9i5kpsp0iun0ktmqgab125ti8\contents\Lib\site-packages\tensorflow\include -IC:\b\s\w\ir\x\w\llvm_build\include -IC:\b\s\w\ir\x\w\llvm-llvm-project\llvm\include -IC:\b\s\w\ir\x\w\llvm-llvm-project\clang-tools-extra\clang-include-fixer\tool\.. -imsvcC:\b\s\w\ir\x\w\zlib_install_target\include -imsvcC:\b\s\w\ir\x\w\zstd_install\include /DWIN32 /D_WINDOWS   /Zc:inline /Zc:__cplusplus /Oi /Brepro /bigobj /permissive- /W4  -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported /Gw -no-canonical-prefixes /O2 /Ob2  -std:c++17 -MT  /EHs-c- /GR- -UNDEBUG /showIncludes /Fotools\clang\tools\extra\clang-include-fixer\tool\CMakeFiles\clang-include-fixer.dir\ClangIncludeFixer.cpp.obj /Fdtools\clang\tools\extra\clang-include-fixer\tool\CMakeFiles\clang-include-fixer.dir\ -c -- C:\b\s\w\ir\x\w\llvm-llvm-project\clang-tools-extra\clang-include-fixer\tool\ClangIncludeFixer.cpp
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang-tools-extra\clang-include-fixer\tool\ClangIncludeFixer.cpp:11:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang-tools-extra\clang-include-fixer\tool\..\IncludeFixer.h:15:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/Sema/ExternalSemaSource.h:15:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/AST/ExternalASTSource.h:18:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/AST/DeclBase.h:18:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/AST/DeclarationName.h:18:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/Basic/IdentifierTable.h:18:
In file included from C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/Basic/Builtins.h:63:
C:\b\s\w\ir\x\w\llvm_build\tools\clang\include\clang/Basic/Builtins.inc(151,1): error: redefinition of enumerator 'BI_alloca'
  151 | LANGBUILTIN(_alloca, "v*z", "n", ALL_MS_LANGUAGES)
      | ^
C:\b\s\w\ir\x\w\llvm_build\tools\clang\include\clang/Basic/Builtins.inc(15,54): note: expanded from macro 'LANGBUILTIN'
   15 | #  define LANGBUILTIN(ID, TYPE, ATTRS, BUILTIN_LANG) BUILTIN(ID, TYPE, ATTRS)
      |                                                      ^
C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/Basic/Builtins.h(62,34): note: expanded from macro 'BUILTIN'
   62 | #define BUILTIN(ID, TYPE, ATTRS) BI##ID,
      |                                  ^
<scratch space>(72,1): note: expanded from here
   72 | BI_alloca
      | ^
C:\b\s\w\ir\x\w\llvm_build\tools\clang\include\clang/Basic/Builtins.inc(150,1): note: previous definition is here
  150 | LIBBUILTIN(alloca, "v*z", "fn", STDLIB_H, ALL_GNU_LANGUAGES)
      | ^
C:\b\s\w\ir\x\w\llvm_build\tools\clang\include\clang/Basic/Builtins.inc(11,61): note: expanded from macro 'LIBBUILTIN'
   11 | #  define LIBBUILTIN(ID, TYPE, ATTRS, HEADER, BUILTIN_LANG) BUILTIN(ID, TYPE, ATTRS)
      |                                                             ^
C:\b\s\w\ir\x\w\llvm-llvm-project\clang\include\clang/Basic/Builtins.h(62,34): note: expanded from macro 'BUILTIN'
   62 | #define BUILTIN(ID, TYPE, ATTRS) BI##ID,
      |                                  ^
<scratch space>(71,1): note: expanded from here
   71 | BI_alloca
      | ^
```

clang/include/clang/Basic/Builtins.h

@@ -20,6 +20,10 @@
 #include "llvm/ADT/StringRef.h"
 #include <cstring>
 
+// VC++ defines 'alloca' as an object-like macro, which interferes with our
+// builtins.
+#undef alloca
+
 namespace clang {
 class TargetInfo;
 class IdentifierTable;