[OpenMP][Clang][BugFix] Split declares and math functions inclusion.

Summary: This patches fixes an issue in which the __clang_cuda_cmath.h header is being included even when cmath or math.h headers are not included.

Reviewers: jdoerfert, ABataev, hfinkel, caomhin, tra

Reviewed By: tra

Subscribers: tra, mgorny, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61765

llvm-svn: 360626