[OpenMP][OMPIRBuilder] Mirgrate getName from clang to OMPIRBuilder

This change moves the getName function from clang and moves the separator class
members from CGOpenMPRuntime into OMPIRBuilder. Also enusre all the getters
in the config class are const.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D137725