wrappers: Add a wrapper for clang-scan-deps

This picks the architecture by looking for a triple prefix in the
nested command, and passes it as an explicit flag, to let
clang-scan-deps find it.

We also pass -stdlib=libc++, to let it find the libc++ headers
as expected.

install-wrappers.sh

@@ -126,6 +126,7 @@ if [ -n "$HOST" ] && [ -n "$EXEEXT" ]; then
     done
 fi
 $CC wrappers/clang-target-wrapper.c -o "$PREFIX/bin/clang-target-wrapper$EXEEXT" -O2 -Wl,-s $WRAPPER_FLAGS
+$CC wrappers/clang-scan-deps-wrapper.c -o "$PREFIX/bin/clang-scan-deps-wrapper$EXEEXT" -O2 -Wl,-s $WRAPPER_FLAGS
 $CC wrappers/llvm-wrapper.c -o "$PREFIX/bin/llvm-wrapper$EXEEXT" -O2 -Wl,-s $WRAPPER_FLAGS
 if [ -n "$EXEEXT" ]; then
     # For Windows, we should prefer the executable wrapper, which also works
@@ -141,6 +142,7 @@ for arch in $ARCHS; do
         for exec in clang clang++ gcc g++ c++ as; do
             ln -sf clang-target-wrapper$CTW_SUFFIX $arch-w64-$target_os-$exec$CTW_LINK_SUFFIX
         done
+        ln -sf clang-scan-deps-wrapper$CTW_SUFFIX $arch-w64-$target_os-clang-scan-deps$CTW_LINK_SUFFIX
         for exec in addr2line ar ranlib nm objcopy readelf size strings strip llvm-ar llvm-ranlib; do
             if [ -n "$EXEEXT" ]; then
                 link_target=llvm-wrapper