[libclang] Fix error handler in translateSourceLocation.

Given an invalid SourceLocation, translateSourceLocation will call
clang_getNullLocation, and then do nothing with the result. But
clang_getNullLocation has no side effects: it just constructs and
returns a null CXSourceLocation value.

Surely the intention was to //return// that null CXSourceLocation to
the caller, instead of throwing it away and pressing on anyway.

Reviewed By: miyuki

Differential Revision: https://reviews.llvm.org/D104442

clang/tools/libclang/CXSourceLocation.h

@@ -29,7 +29,7 @@ static inline CXSourceLocation
 translateSourceLocation(const SourceManager &SM, const LangOptions &LangOpts,
                         SourceLocation Loc) {
   if (Loc.isInvalid())
-    clang_getNullLocation();
+    return clang_getNullLocation();
 
   CXSourceLocation Result = { { &SM, &LangOpts, },
                               Loc.getRawEncoding() };