[Bugpoint] Use clang by default.

[Bugpoint] Use clang by default.

We now rely on gcc only if either of the following is true:
1) -gcc option is passed by the user
2) clang is not found in the default path.

Differential Revision:	 http://reviews.llvm.org/D13642

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250318 91177308-0d34-0410-b5e6-96231b3b80d8