[opaque pointer type] Pass GlobalAlias the actual pointer type rather than...

[opaque pointer type] Pass GlobalAlias the actual pointer type rather than decomposing it into pointee type + address space

Many of the callers already have the pointer type anyway, and for the
couple of callers that don't it's pretty easy to call PointerType::get
on the pointee type and address space.

This avoids LLParser from using PointerType::getElementType when parsing
GlobalAliases from IR.

llvm-svn: 236160