Force C89 for c-index-test.c

MSVC supports only C89, so it is important to keep c-index-test.c
buildable with C89.  However, Clang defaults to C99, so while building
Clang with Clang one can introduce C99 constructs into c-index-test.c
without noticing.

Thanks to Nakamura Takumi for helping with MSVC bits.

llvm-svn: 167561