Merging r295313:

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r295313 | stulova | 2017-02-16 04:49:29 -0800 (Thu, 16 Feb 2017) | 9 lines

[OpenCL][Doc] Added OpenCL vendor extension description to user manual doc

Added description of a new feature that allows to specify
vendor extension in flexible way using compiler pragma instead
of modifying source code directly (committed in clang@r289979).

Review: D29829


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llvm-svn: 295340