[AMDGPU] Restructure code object metadata creation

  - Rename runtime metadata -> code object metadata
  - Make metadata not flow
  - Switch enums to use ScalarEnumerationTraits
  - Cleanup and move AMDGPUCodeObjectMetadata.h to AMDGPU/MCTargetDesc
  - Introduce in-memory representation for attributes
  - Code object metadata streamer
  - Create metadata for isa and printf during EmitStartOfAsmFile
  - Create metadata for kernel during EmitFunctionBodyStart
  - Finalize and emit metadata to .note during EmitEndOfAsmFile
  - Other minor improvements/bug fixes

Differential Revision: https://reviews.llvm.org/D29948

llvm-svn: 298552