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

[Wenju He via cfe-commits]

================
@@ -0,0 +1,16 @@
+set(CLC_AMDGPU_SOURCES
----------------
wenju-he wrote:

`libclc_configure_source_list(CLC_AMDGPU_SOURCES`
and in the macro
```
macro(libclc_configure_source_list CLC_SOURCES)
  set(${CLC_SOURCES} ${ARGN})
```

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