Add pointer types to global named register

This patch adds support for pointer types in global named registers variables.
It'll be lowered as a pair of read/write_register and inttoptr/ptrtoint calls.
Also adds some early checks on types on SemaDecl to avoid the assert.

Tests changed accordingly. (PR19837)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210274 91177308-0d34-0410-b5e6-96231b3b80d8