[ms-inline asm] Use StringRef here, per Jordan's suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161619 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaStmt.cpp b/lib/Sema/SemaStmt.cpp
index 856a602..944fc40 100644
--- a/lib/Sema/SemaStmt.cpp
+++ b/lib/Sema/SemaStmt.cpp
@@ -2890,7 +2890,7 @@
 
   // Empty asm statements don't need to instantiate the AsmParser, etc.
   if (AsmToks.empty()) {
-    std::string AsmString;
+    StringRef AsmString;
     MSAsmStmt *NS =
       new (Context) MSAsmStmt(Context, AsmLoc, /* IsSimple */ true,
                               /* IsVolatile */ true, AsmToks, LineEnds,