[DebugInfo][NFC] Add RawLocationWrapper to wrap location operand metadata [1/x]

RawLocationWrapper wraps the location operand of a debug intrinsic which may be
either (wrapped in MetadataAsValue) a DIArgList, ValueAsMetadata, or an empty
MDTuple. This class lets us avoid duplicating location handling code in a later
patch in this stack.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D145909