[build] only download clang if it is needed
[build] only download clang if it is needed

Summary:
Make sure that we only download the clang tarballs needed to build clang
if:
1. clang is not installed
2. the user intends to build clang

Reviewed By: martintrojer

Differential Revision: D24137085

fbshipit-source-id: bfea1bf02

jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer