[libclc] [libclc] Fix a couple of issues preventing in-tree builds (PR #87505)

[Michał Górny via cfe-commits]

mgorny wrote:

This change broke standalone build against LLVM dylib — it now insists on linking to non-existing static libraries:

```
FAILED: prepare_builtins 
: && /usr/lib/ccache/bin/x86_64-pc-linux-gnu-g++ -march=znver2 --param=l1-cache-size=32 --param=l1-cache-line-size=64 -O2 -pipe -frecord-gcc-switches -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0     -Wl,-rpath-link,  -Wl,--gc-sections CMakeFiles/prepare_builtins.dir/utils/prepare-builtins.cpp.o -o prepare_builtins  -Wl,-rpath,"\$ORIGIN/../lib64:/usr/lib/llvm/17/lib64"  -lLLVMBitReader  -lLLVMBitWriter  -lLLVMCore  -lLLVMIRReader  /usr/lib/llvm/17/lib64/libLLVMSupport.a  -lrt  -ldl  -lm  /usr/lib64/libz3.so  /usr/lib64/libz.so  /usr/lib64/libzstd.so  /usr/lib64/libtinfo.so  /usr/lib/llvm/17/lib64/libLLVMDemangle.a && :
/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lLLVMBitReader: No such file or directory
/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lLLVMBitWriter: No such file or directory
/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lLLVMCore: No such file or directory
/usr/lib/gcc/x86_64-pc-linux-gnu/13/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lLLVMIRReader: No such file or directory
collect2: error: ld returned 1 exit status
```

Given that it apparently doesn't fix in-tree builds fully, please revert and let's figure out how to do it properly without breaking its primary use case.

https://github.com/llvm/llvm-project/pull/87505