build-compiler-rt: Make sure that compiler-rt doesn't pick up an external llvm-config

We normally don't install all tools (including llvm-config), or we might
have stripped them out, but the system distro might have installed a copy
of llvm-config (for a different unrelated installation of llvm).

Some compiler-rt runtimes (such as xray and orc) use this for finding
the main llvm headers.

When building compiler-rt, we really don't want it to pick up any random
version of llvm headers found on the system, but keep using the matching
headers from the monorepo instead.

Adding the include directory of a system provided set of llvm headers
is especially fatal if that directory isn't a subdirectory containing
only llvm headers, but contains other system headers too.

In particular, on msys2, the include directory (pointed at by "llvm-config
--includedir", and also set by including LLVMConfig.cmake which is
pointed at by "llvm-config --cmakedir") is the main system include
directory. Including this can also get headers for a system install
of libc++ headers (for the clang64 msys2 distro) - and using libc++
headers with a second set of libc++ headers in the include path is
fatal (due to include_next not finding the plain C header but finding
a second libc++ header).

Just try to make sure that we don't pick up any unintended headers.

build-compiler-rt.sh

@@ -95,6 +95,7 @@ for arch in $ARCHS; do
         -DCMAKE_C_COMPILER_TARGET=$arch-windows-gnu \
         -DCOMPILER_RT_DEFAULT_TARGET_ONLY=TRUE \
         -DCOMPILER_RT_USE_BUILTINS_LIBRARY=TRUE \
+        -DLLVM_CONFIG_PATH="" \
         -DSANITIZER_CXX_ABI=libc++ \
         $SRC_DIR
     $BUILDCMD ${CORES+-j$CORES}