Fix an invalid Twine use spotty by abbeyj, it isn't safe to use Twine

temporaries (this is one reason I'm nervous about propagating their use beyond
particularly performance critical places).

llvm-svn: 93981

clang/lib/Frontend/InitPreprocessor.cpp

@@ -137,7 +137,10 @@ static void DefineFloatMacros(MacroBuilder &Builder, llvm::StringRef Prefix,
                "1.79769313486231580793728971405301e+308L",
                "1.18973149535723176508575932662800702e+4932L");
 
-  llvm::Twine DefPrefix = "__" + Prefix + "_";
+  llvm::SmallString<32> DefPrefix;
+  DefPrefix = "__";
+  DefPrefix += Prefix;
+  DefPrefix += "_";
 
   Builder.defineMacro(DefPrefix + "DENORM_MIN__", DenormMin);
   Builder.defineMacro(DefPrefix + "HAS_DENORM__");