grouper: use a prebuilt clang

results:
Stock clang:
16587
16787
16503
16751
16479
--------+
83107/5
--------
16621,4

msm clang:
16655
16936
16810
16830
16958
--------+
84189/5
--------
16837,8

profit = 216,4

Change-Id: I4168599740950c8ec9b494827c5a4e47fedac234