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

[Wenju He via cfe-commits]

================
@@ -0,0 +1,14 @@
+set(CLC_AMDGCN_SOURCES
----------------
wenju-he wrote:

avoid prepending for every line using `list(TRANSFORM`?
```
set(CLC_AMDGCN_SOURCES
  address_space/qualifier.cl
  ...
)

list(TRANSFORM CLC_AMDGCN_SOURCES PREPEND "${CMAKE_CURRENT_SOURCE_DIR}/")
```

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