[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350521
91177308-0d34-0410-b5e6-
96231b3b80d8