[Remarks] Fix usage of enum class

Breaks the build on some compilers:

http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/9720/steps/build%20stage%201/logs/stdio

llvm-svn: 362165
diff --git a/llvm/lib/Remarks/YAMLRemarkSerializer.cpp b/llvm/lib/Remarks/YAMLRemarkSerializer.cpp
index 47fef5d..14de44f 100644
--- a/llvm/lib/Remarks/YAMLRemarkSerializer.cpp
+++ b/llvm/lib/Remarks/YAMLRemarkSerializer.cpp
@@ -154,7 +154,7 @@
 
 YAMLSerializer::YAMLSerializer(raw_ostream &OS, UseStringTable UseStringTable)
     : Serializer(OS), YAMLOutput(OS, reinterpret_cast<void *>(this)) {
-  if (UseStringTable == UseStringTable::Yes || RemarksYAMLStringTable)
+  if (UseStringTable == remarks::UseStringTable::Yes || RemarksYAMLStringTable)
     StrTab.emplace();
 }