Fix Windows build breakage from #18490

[tkelman]

Need to patch LLVM's makefiles to create DestSharedLibDir during install-llvm if the value set at that time differs from the value set during compile-llvm.

[tkelman]

cc @vtjnash or @Keno for review, I tested this in clean builds on the 3 main 64 bit arches

error was

make[3]: *** No rule to make target '/home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts/home/Tony/julia/usr/bin', needed by '/home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts/home/Tony/julia/usr/bin/LLVM-3.7.dll'.  Stop.
make[2]: *** [/home/Tony/julia/deps/srccache/llvm-3.7.1/Makefile.rules:880: install] Error 1
make[1]: *** [/home/Tony/julia/deps/llvm.mk:526: /home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts.tgz] Error 2
make: *** [Makefile:81: julia-deps] Error 2

[Keno]

Looks fine to me.

[tkelman]

Thanks. OSX Travis failure was an unrelated freeze, and LLVM there gets used from homebrew-julia instead of built from source.