[Clang][OpenMP][IRBuilder] Move registerTargetGlobalVariable &...

[Clang][OpenMP][IRBuilder] Move registerTargetGlobalVariable & getAddrOfDeclareTargetVar into the OMPIRBuilder

This change tries to move registerTargetglobalVariable and
getAddrOfDeclareTargetVar out of Clang's CGOpenMPRuntime
and into the OMPIRBuilder for shared use with MLIR's OpenMPDialect
and Flang (or other languages that may want to utilise it).

This primarily does this by trying to hoist the Clang specific
types into arguments or callback functions in the form of
lambdas, replacing it with LLVM equivelants and
utilising shared OMPIRBuilder enumerators for
the clauses, rather than Clang's own variation.

Reviewers: jsjodin, jdoerfert

Differential Revision: https://reviews.llvm.org/D149162