Windows build fails if llvm 9 or later is installed

[asajeffrey]

On windows, we install llvm 8.0.0, and use it in bindgen for mozjs. Unfortunately if llvm 9 is isntalled as the system llvm, its includes get picked up, resulting in build errors:

C:\Program Files\LLVM\lib\clang\9.0.0\include\emmintrin.h:2308:19: error: use of undeclared identifier '__builtin_ia32_pavgb128'
C:\Program Files\LLVM\lib\clang\9.0.0\include\emmintrin.h:2328:19: error: use of undeclared identifier '__builtin_ia32_pavgw128'
C:\Users\ajeffrey\github\asajeffrey\servo\target\debug\build\mozjs_sys-5d322e9a9264f323\out\dist/include\mozilla/Assertions.h:219:1: warning: function declared 'noreturn' should not return [-Winvalid-noreturn], err: false
C:\Program Files\LLVM\lib\clang\9.0.0\include\emmintrin.h:2308:19: error: use of undeclared identifier '__builtin_ia32_pavgb128', err: true
C:\Program Files\LLVM\lib\clang\9.0.0\include\emmintrin.h:2328:19: error: use of undeclared identifier '__builtin_ia32_pavgw128', err: true
thread 'main' panicked at 'Should generate JSAPI bindings OK: ()', src\libcore\result.rs:1188:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Build FAILED in 0:00:13