[Plugins/Process] Remove direct use of ClangASTContext from InferiorCallPOSIX

Summary:
InferiorCallPOSIX directly grabs a ClangASTContext from the Target it
has and does no error checking. I don't think these functions have a
reason to know about clang specifically. Additionally, using
`GetScratchTypeSystemForLanguage` forces us to do error checking since
it returns an Expected.

Differential Revision: https://reviews.llvm.org/D67427

llvm-svn: 371654