[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. Then it ends up with
`clang++: error: no such file or directory: '/repo/llvm/build-all/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/fuzzer/libcxx_fuzzer_x86_64/lib/libc++.a'`
So there are more paths that are wrong if this is the way forward. Maybe something like this:
```
-    partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX} ${arch})
-    partially_link_libcxx(fuzzer_interceptors ${LIBCXX_${arch}_PREFIX} ${arch})
-    partially_link_libcxx(fuzzer ${LIBCXX_${arch}_PREFIX} ${arch})
+   partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX}/build ${arch})
+   partially_link_libcxx(fuzzer_interceptors ${LIBCXX_${arch}_PREFIX}/build ${arch})
+   partially_link_libcxx(fuzzer ${LIBCXX_${arch}_PREFIX}/build ${arch})
```

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