[libc++][test] Improves src include paths. (llvm#86864)

Instead of including a relative path use an absolute path based on the
available lit substitution. This makes it easier to understand what is
included and moving the test to a different directory level no longer
breaks the test.

This is based on a question by @EricWF in
llvm#82113.

libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp

@@ -9,7 +9,7 @@
 // UNSUPPORTED: c++03, c++11, c++14
 // UNSUPPORTED: availability-filesystem-missing
 // UNSUPPORTED: no-filesystem
-// ADDITIONAL_COMPILE_FLAGS: -I %S/../../../../../../src
+// ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src
 
 // This test relies on calling functions from the libcxx internal headers
 // of <filesystem>; the Windows implementation uses different

libcxx/test/libcxx/input.output/filesystems/convert_file_time.pass.cpp

@@ -13,7 +13,7 @@
 
 // typedef TrivialClock file_time_type;
 
-// ADDITIONAL_COMPILE_FLAGS: -I %S/../../../../src -Wno-macro-redefined
+// ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src -Wno-macro-redefined
 
 #include <cassert>
 #include <chrono>

libcxx/test/libcxx/time/time.zone/time.zone.db/rules.pass.cpp

@@ -17,7 +17,7 @@
 // Tests the IANA database rules parsing and operations.
 // This is not part of the public tzdb interface.
 // The test uses private implementation headers.
-// ADDITIONAL_COMPILE_FLAGS: -I %S/../../../../../src/include
+// ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src/include
 
 #include <chrono>
 #include <fstream>

libcxx/test/libcxx/time/time.zone/time.zone.db/zones.pass.cpp

@@ -17,7 +17,7 @@
 // Tests the IANA database zones parsing and operations.
 // This is not part of the public tzdb interface.
 // The test uses private implementation headers.
-// ADDITIONAL_COMPILE_FLAGS: -I %S/../../../../../src/include
+// ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src/include
 
 #include <cassert>
 #include <chrono>