[libclc] [libclc] Use custom CMake handling to overhaul libclc compilation (PR #185247)

[Wenju He via cfe-commits]

wenju-he wrote:

I have just tested your branch and compared with commit prior to the change. Bitcode and spirv files looks good, but there are a few symbols missing in the new library. It is probably that there is new filename list needs updating:
```
function @_Z24__clc_work_group_barrieri21__CLC_MemorySemantics exists only in right module
function @_Z14work_group_alli exists only in right module
function @_Z14work_group_anyi exists only in right module
function @_Z20work_group_broadcastim exists only in right module
function @_Z20work_group_broadcastimm exists only in right module
function @_Z20work_group_broadcastimmm exists only in right module
function @_Z20work_group_broadcastjm exists only in right module
function @_Z20work_group_broadcastjmm exists only in right module
function @_Z20work_group_broadcastjmmm exists only in right module
function @_Z20work_group_broadcastlm exists only in right module
function @_Z20work_group_broadcastlmm exists only in right module
function @_Z20work_group_broadcastlmmm exists only in right module
function @_Z20work_group_broadcastmm exists only in right module
function @_Z20work_group_broadcastmmm exists only in right module
function @_Z20work_group_broadcastmmmm exists only in right module
function @_Z20work_group_broadcastfm exists only in right module
function @_Z20work_group_broadcastfmm exists only in right module
function @_Z20work_group_broadcastfmmm exists only in right module
function @_Z20work_group_broadcastdm exists only in right module
function @_Z20work_group_broadcastdmm exists only in right module
function @_Z20work_group_broadcastdmmm exists only in right module
function @_Z9mem_fencej exists only in right module
function @_Z14read_mem_fencej exists only in right module
function @_Z15write_mem_fencej exists only in right module
function @_Z18get_sub_group_sizev exists only in right module
function @_Z22get_max_sub_group_sizev exists only in right module
function @_Z28__clc_get_max_sub_group_sizev exists only in right module
function @_Z18get_num_sub_groupsv exists only in right module
function @_Z27get_enqueued_num_sub_groupsv exists only in right module
function @_Z33__clc_get_enqueued_num_sub_groupsv exists only in right module
function @_Z16get_sub_group_idv exists only in right module
function @_Z22get_sub_group_local_idv exists only in right module
function @_Z28__clc_get_sub_group_local_idv exists only in right module
function @_Z13sub_group_alli exists only in right module
function @_Z19__clc_sub_group_alli exists only in right module
function @_Z13sub_group_anyi exists only in right module
function @_Z19__clc_sub_group_anyi exists only in right module
function @_Z17sub_group_barrierj12memory_scope exists only in right module
function @_Z23__clc_sub_group_barrier21__CLC_MemorySemanticsi exists only in right module
function @_Z17sub_group_barrierj exists only in right module
function @_Z23__clc_get_global_offsetj exists only in right module
function @_Z12get_work_dimv exists only in right module
```

https://github.com/llvm/llvm-project/pull/185247