[UB] When attaching empty strings to the AST, use an empty StringRef

rather than forcing the bump pointer allocator to produce a viable
pointer. This also fixes UB when we would try to memcpy from the null
incoming StringRef.

llvm-svn: 243947

clang/lib/AST/Stmt.cpp

@@ -724,6 +724,8 @@ MSAsmStmt::MSAsmStmt(const ASTContext &C, SourceLocation asmloc,
 }
 
 static StringRef copyIntoContext(const ASTContext &C, StringRef str) {
+  if (str.empty())
+    return StringRef();
   size_t size = str.size();
   char *buffer = new (C) char[size];
   memcpy(buffer, str.data(), size);