[DebugInfo] Expand ability to load 2-byte addresses in dwarf sections

Some dwarf loaders in LLVM are hard-coded to only accept 4-byte and 8-byte address sizes. This patch generalizes acceptance into `DWARFContext::isAddressSizeSupported` and provides a common way to generate rejection errors.

The MSP430 target has been given new tests to cover dwarf loading cases that previously failed due to 2-byte addresses.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D111953