support using an existing clang installation

add an option --host-clang which will skip building llvm,
and instead provide symlinks and wrappers for clang and
llvm tools.

Currently the clang version is detected, but it does not
affect the llvm version that will be downloaded and used.

build-compiler-rt.sh

@@ -81,6 +81,8 @@ else
 fi
 
 cd llvm-project/compiler-rt
+# Use a staging directory in case parts of the resource dir are immutable
+WORKDIR=$(mktemp -d); trap "rm -rf $WORKDIR" 0
 
 for arch in $ARCHS; do
     if [ -n "$SANITIZERS" ]; then
@@ -123,10 +125,16 @@ for arch in $ARCHS; do
         -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS" \
         $SRC_DIR
     cmake --build . ${CORES:+-j${CORES}}
-    cmake --install .
+    cmake --install . --prefix "${WORKDIR}/install"
     mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
     if [ -n "$SANITIZERS" ]; then
-        mv "$CLANG_RESOURCE_DIR/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
+        mv "${WORKDIR}/install/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
     fi
     cd ..
 done
+
+if [ -h "$CLANG_RESOURCE_DIR/include" ]; then
+    # symlink to system headers - skip copy
+    rm -rf ${WORKDIR}/install/include
+fi
+cp -r ${WORKDIR}/install/. $CLANG_RESOURCE_DIR