Merging r368552:

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r368552 | stulova | 2019-08-12 12:44:07 +0200 (Mon, 12 Aug 2019) | 12 lines

[OpenCL] Fix lang mode predefined macros for C++ mode.

In C++ mode we should only avoid adding __OPENCL_C_VERSION__,
all other predefined macros about the language mode are still
valid.

This change also fixes the language version check in the
headers accordingly.

Differential Revision: https://reviews.llvm.org/D65941


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llvm-svn: 368844