[Test] Remove substitution for clang

clang should not be used in the llvm tests.  The topic was discussed in this
thread:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140428/214905.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207976 91177308-0d34-0410-b5e6-96231b3b80d8