Summary:
This PR uses https://github.com/llvm/llvm-project/pull/185243 to
overhaul compilation of libclc. This brings libclc to the same kind of
compilation flow that the other GPU libraries use (compiler-rt, libc,
libc++, openmp, flang-rt).
The main brunt of this change is simply changing the SOURCES files to
CMake variables and altering the compilation. Now that these are
standard CMake libraries we do not need to bother redefining custom
library handling and targets.
This builds as a static library, which we then consume with `llvm-link`
which converts it into a single `.bc` bitcode file similarly to before.
The final result is then optimized all together.
Hopefully this doesn't break anything.
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