Set PreprocessorOpts.GeneratePreamble=true in PrecompiledPreamble.

Summary:
It was previsouly set only in ASTUnit, but it should be set for all client of
PrecompiledPreamble.

Reviewers: erikjv, bkramer, klimek

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38617

llvm-svn: 315212