[clang][amdgpu] - Choose when to promote VarDecl to address space 4.

There are instances where clang codegen creates stores to
address space 4 in ctors, which causes a crash in llc.
This store was being optimized out at opt levels > 0.

For example:

pragma omp declare target
static  const double log_smallx = log2(smallx);
pragma omp end declare target

This patch ensures that any global const that does not
have constant initialization stays in address space 1.

Note - a second patch is in the works where all global
constants are placed in address space 1 during
codegen and then the opt pass InferAdressSpaces
will promote to address space 4 where necessary.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D115661