xcode-cross: Execute clang within the xctoolchain directory

This makes it pick up the libc++ headers from within there,
instead of the ones that were shipped with the vanilla clang,
allowing us to remove the libc++ headers shipped with clang.

Clang still uses the compiler-rt files from the directory
next to the actual clang executable though.

Dockerfile

@@ -19,11 +19,7 @@ RUN set -x \
   && mv bin/clang-11 bin/clang bin/clang++ bin/dsymutil bin/llvm-nm bin-new \
   && rm -rf bin \
   && mv bin-new bin \
-  && mkdir include-new \
-  && mv include/c++ include-new \
-  && rm -rf include \
-  && mv include-new include \
-  && rm -rf lib/*.a lib/*.so lib/*.so.* lib/clang/11.0.0/lib/linux
+  && rm -rf lib/*.a lib/*.so lib/*.so.* lib/clang/11.0.0/lib/linux include
 
 RUN set -x \
   && git clone https://github.com/facebook/xcbuild.git xcbuild-src \

setup-toolchain.sh

@@ -82,10 +82,9 @@ if [ -z "\$SYSROOT_SET" ]; then
 fi
 export PATH=$CCTOOLS/bin:\$PATH
 if [ "\$TOOL" = "clang++" ] || [ "\$TOOL" = "c++" ] || [ "\$TOOL" = "g++" ]; then
-	EXE=$CLANG/bin/clang++
-else
-	EXE=$CLANG/bin/clang
+	ARGS+=(--driver-mode=g++)
 fi
+EXE=\$(dirname \$(readlink -f \$0))/clang-exe
 \$EXE "\${ARGS[@]}"
 EOF
 chmod a+x clang
@@ -99,6 +98,7 @@ ln -sf clang g++
 
 ln -s $CLANG/bin/dsymutil .
 ln -s $CLANG/bin/llvm-nm nm
+ln -s $CLANG/bin/clang clang-exe
 
 cd ../lib
 mkdir -p $CLANG/lib/arc