[Clang][OpenMP][OMPBuilder] Moving OMP allocation and cache creation code to OMPBuilderCBHelpers

Summary:
Modified the OMPBuilderCBHelpers in the following ways:
- Moved location of class definition and deleted all constructors
- Moved OpenMP-specific address allocation of local variables
- Moved threadprivate variable creation for the current thread

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79676