[OpenMP] Compilation error fix on const char*

Summary:
This line
(https://github.com/llvm-mirror/openmp/blob/0ed912c7a798f5c4f65f8bb6b492e07fab7f4cea/runtime/src/kmp_gsupport.cpp#L1459)
added in D45327 (rL330282) causes a compilation failure.

Reviewers: jlpeyton

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D45786

llvm-svn: 330299