Fix incorrect cast in VisitSYCLUniqueStableNameExpr

Fix incorrect cast in VisitSYCLUniqueStableNameExpr

Clang language-level address spaces and LLVM pointer address spaces are
not the same thing (even though they will both have a numeric value of
zero in many cases). LangAS is a enum class to avoid implicit conversions,
but eba69b59d1a30dead07da2c279c8ecfd2b62ba9f avoided the compiler error by
adding a `static_cast<>`. While touching this code, simplify it by using
CreatePointerBitCastOrAddrSpaceCast() which is already a no-op if the types
match.

This changes the code generation for spir64 to place the globals in
the sycl_global addreds space, which maps to `addrspace(1)`.

Reviewed By: bader

Differential Revision: https://reviews.llvm.org/D138284