[OPENMP]Target region: emit const firstprivates as globals with constant

[OPENMP]Target region: emit const firstprivates as globals with constant
memory.

If the variable with the constant non-scalar type is firstprivatized in
the target region, the local copy is created with the data copying.
Instead, we allocate the copy in the constant memory and avoid extra
copying in the outlined target regions. This global copy is used in the
target regions without loss of the performance.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@355418 91177308-0d34-0410-b5e6-96231b3b80d8