[compiler-rt] [compiler-rt] Use installed libc++(abi) for tests instead of build tree (PR #115077)

[Björn Pettersson via llvm-commits]

bjope wrote:

> > > But I guess the goal was to use a path to the just installed version here (from the DESTDIR)?
> > 
> > 
> > Or should the isystem paths setup by compiler-rt/lib/fuzzer/CMakeLists.txt use `.../build/include/c++/v1` now?
> 
> Btw, it doesn't seem to be enough to change the isystem paths. 

Maybe it is more correct to do:
```
- set(LIBCXX_${arch}_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_fuzzer_${arch})
+ set(LIBCXX_${arch}_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_fuzzer_${arch}/build)
```

https://github.com/llvm/llvm-project/pull/115077