[lldb][NFC] Pass Stream& to ToXML methods in RegisterFlags

As suggested in a review of some new code for this file, Stream
is more general. The code does not need to know that it's backed
by a string.

lldb/include/lldb/Target/RegisterFlags.h

@@ -15,7 +15,7 @@
 
 namespace lldb_private {
 
-class StreamString;
+class Stream;
 class Log;
 
 class RegisterFlags {
@@ -56,7 +56,7 @@ class RegisterFlags {
     /// Output XML that describes this field, to be inserted into a target XML
     /// file. Reserved characters in field names like "<" are replaced with
     /// their XML safe equivalents like "&gt;".
-    void ToXML(StreamString &strm) const;
+    void ToXML(Stream &strm) const;
 
     bool operator<(const Field &rhs) const {
       return GetStart() < rhs.GetStart();
@@ -119,7 +119,7 @@ class RegisterFlags {
   std::string AsTable(uint32_t max_width) const;
 
   // Output XML that describes this set of flags.
-  void ToXML(StreamString &strm) const;
+  void ToXML(Stream &strm) const;
 
 private:
   const std::string m_id;

lldb/source/Target/RegisterFlags.cpp

@@ -190,7 +190,7 @@ std::string RegisterFlags::AsTable(uint32_t max_width) const {
   return table;
 }
 
-void RegisterFlags::ToXML(StreamString &strm) const {
+void RegisterFlags::ToXML(Stream &strm) const {
   // Example XML:
   // <flags id="cpsr_flags" size="4">
   //   <field name="incorrect" start="0" end="0"/>
@@ -213,7 +213,7 @@ void RegisterFlags::ToXML(StreamString &strm) const {
   strm.Indent("</flags>\n");
 }
 
-void RegisterFlags::Field::ToXML(StreamString &strm) const {
+void RegisterFlags::Field::ToXML(Stream &strm) const {
   // Example XML:
   // <field name="correct" start="0" end="0"/>
   strm.Indent();