[CTU] Make loadExternalAST return with non nullptr on success

Summary:
In loadExternalAST we return with either an error or with a valid
ASTUnit pointer which should not be a nullptr.
This prevents in the call site any superfluous check for being a nullptr.

Reviewers: xazax.hun, a_sidorin, Szelethus, balazske

Subscribers: rnkovacs, dkrupp, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D55280

llvm-svn: 350521