Support '#pragma once' in headers when using PCH

    
The '#pragma once' directive was erroneously ignored when encountered
in the header-file specified in generate-PCH-mode. This resulted in
compile-time errors in some cases with legal code, and also a misleading
warning being produced.

Patch by Warren Ristow!

Differential Revision: http://reviews.llvm.org/D19815


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276653 91177308-0d34-0410-b5e6-96231b3b80d8