[Modules] More descriptive diagnostics for misplaced import directive

[Modules] More descriptive diagnostics for misplaced import directive

If an import directive was put into wrong context, the error message was obscure,
complaining on misbalanced braces. To get more descriptive messages, annotation
tokens related to modules are processed where they must not be seen.

Differential Revision: http://reviews.llvm.org/D11844

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@248085 91177308-0d34-0410-b5e6-96231b3b80d8