Remove some std stream usage from Support and TableGen

LLVM's coding standards recommend raw_ostream and MemoryBuffer for
reading and writing text.

This has the side effect of allowing clang to compile more of Support
and TableGen in the Microsoft C++ ABI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187826 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/TableGen/AsmMatcherEmitter.cpp b/utils/TableGen/AsmMatcherEmitter.cpp
index 840c9aa..468ce1c 100644
--- a/utils/TableGen/AsmMatcherEmitter.cpp
+++ b/utils/TableGen/AsmMatcherEmitter.cpp
@@ -2069,7 +2069,8 @@
   OS << "  if (A == B)\n";
   OS << "    return true;\n\n";
 
-  std::stringstream SS;
+  std::string OStr;
+  raw_string_ostream SS(OStr);
   unsigned Count = 0;
   SS << "  switch (A) {\n";
   SS << "  default:\n";