Test the fwd decl generator with all headers in the include path.
Remove debug printout. Run in verify mode.
Reduce the iteration to only STL (at least on Ubu is the first include path.)
Move the diagnostic client setup before parser's initialization.

This would allow us to produce diagnostics in the invocation args, such as
 -DFOO=''
Fixes ROOT-6538.
Move the code further up.
Follow recent llvm interface changes getPtr()->get() .
getTargetOpts -> TargetOpts.
setForcedLangOptions -> adjust.
MSCVersion -> MSCompatibilityVersion .
Adapt to interface changes in clang diagnostics.
llvm::OwningPtr to std::unique_ptr.
Sanitizer opts take less options.
llvm::errc -> std::errc.
getPtr -> get.
Method takes a llvm::ArrayRef.
Force printing bool instead of _Bool.
Try #include the generated files by separate instance.
Update last known good llvm.
Disable #inclusion of the generated fwd decl headers.