[AMDGPU] Update assembler for HSA Code Object v3

Update AMDGPU assembler syntax behind the code-object-v3 feature:

* Replace/rename most AMDGPU assembler directives/symbols and document them.
* Provide more diagnostics (e.g. values out of range, missing values, repeated
  values).
* Provide path for backwards compatibility, even with underlying descriptor
  changes.

Differential Revision: https://reviews.llvm.org/D47736

llvm-svn: 335281