Update assembler syntax for code object v3 to match https://reviews.llvm.org/D47566
- Replace/rename most AMDGPU assembler directives/symbols and document them
- Provide more diagnostics (e.g. for values out of range, missing required values, repeated values, mismatching command-line options)
The new syntax should allow us to remain backwards compatible, even if the underlying descriptor changes.
GroupSegmentFixedSize -> GROUP_SEGMENT_FIXED_SIZE