[OPENMP 4.0] Codegen for 'declare simd' directive.

OpenMP 4.0 adds support for elemental functions using declarative
directive '#pragma omp declare simd'. Patch adds mangling for simd
functions in accordance with
https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&do=view&target=VectorABI.txt

llvm-svn: 268721