[WebAssembly] Preserve debug frame base information through register coloring

2 fixes:

Register coloring can re-assign virtual registers. When the frame base register
is colored, update the DwarfFrameBase accordingly When the frame base register
is stackified, do not attempt to encode DW_AT_frame_base as a local In the
future we will presumably want to handle this case better but for now we can
emit worse debug info rather than crashing.

Differential Revision: https://reviews.llvm.org/D73581