[DirectX backend] Support global ctor for DXILBitcodeWriter.

1. Save typed pointer type for GlobalVariable/Function instead of the ObjectType.
   This will allow use GlobalVariable/Function as value.
2. Save target type for global ctors for Constant.
3. In DXILBitcodeWriter::getTypeID, check PointerMap first for Constant case.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D133283