Use LLVM_ATTRIBUTE_UNUSED to hide gcc 5.1 unused variable warning.

Suggestion by David Blaikie!

llvm-svn: 244326

clang/lib/Tooling/CompilationDatabase.cpp

@@ -325,7 +325,7 @@ namespace tooling {
 // This anchor is used to force the linker to link in the generated object file
 // and thus register the JSONCompilationDatabasePlugin.
 extern volatile int JSONAnchorSource;
-int JSONAnchorDest = JSONAnchorSource;
+static int LLVM_ATTRIBUTE_UNUSED JSONAnchorDest = JSONAnchorSource;
 
 } // end namespace tooling
 } // end namespace clang