Merging r276653:

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r276653 | ssrivastava | 2016-07-25 10:17:06 -0700 (Mon, 25 Jul 2016) | 11 lines

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

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llvm-svn: 277075