[tests] Add back stdc macros I accidentally refactored out.

 - Patch by Michael van der Westhuizen:
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r174404 accidentally removed stdc format, limit and constant macros from the Linux test runner logic.  This small patch re-adds the macros.

Making this change fixes the following tests on Linux:
 - depr/depr.c.headers/inttypes_h.pass.cpp
 - depr/depr.c.headers/stdint_h.pass.cpp
 - input.output/file.streams/c.files/cinttypes.pass.cpp
 - language.support/cstdint/cstdint.syn/cstdint.pass.cpp
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llvm-svn: 174722