Support/MemoryBuffer: Replace all uses of std::string *ErrMsg with error_code &ec. And fix clients.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121379 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/system_error.cpp

@@ -96,6 +96,15 @@ system_category() {
   return s;
 }
 
+const error_category&
+posix_category() {
+#ifdef LLVM_ON_WIN32
+  return generic_category();
+#else
+  return system_category();
+#endif
+}
+
 // error_condition
 
 std::string