Fix assert "DeclRefExpr for Decl not entered in LocalDeclMap?"

Currently compiler assert when passing variable "memspace" in
omp_init_allocator.

omp_allocator_handle_t alloc=omp_init_allocator(memspace,1,traits)

The problem is memspace is not mapping to the target region.  During
the call to emitAllocatorInit, calls to EmitVarDecl for "alloc", then
emit initialization of "alloc" that cause to assert.

If I understant correct, it is not necessary to emit variable
initialization, since "allocator" is private to target region.

To fix this call CGF.EmitAutoVarAlloca(allocator) instead
CGF.EmitVarDecl(allocator).

Differential Revision: https://reviews.llvm.org/D151743