Revert "grouper: use a prebuilt clang"

This breaks build, because there's something generally wrong in build when prebuilt clang is used that breaks dex preopt execution.

As this depends on fixes that aren't merged, this shouldn't have been merged.

This reverts commit 661ec2b82cf48395f4fb792f59f3073f9b107d56.

Change-Id: Ib129bb51e5af3ef6e7ff3d13d92265246f235c59