Fix wrong error message when compiling C souce code:

Fix wrong error message when compiling C souce code:
Currently emit error for uses_allocators(alloc(traits)):

called object type 'omp_allocator_handle_t' (aka
'enum omp_allocator_handle_t') is not a function or function pointer

To fix this, since "alloc" is Id expresison(spce 5.2), during the parser
(in ParseOpenMP.cpp), using tryParseCXXIdExpression instead of
ParseExpression for C.

Differential Revision: https://reviews.llvm.org/D151517