Fix set-xcode-analyzer to only modify the ExecPath for the analyzer when using --use-xcode-clang.

Turns out that the ExecPath for the ObjC migrator would also get set.

Fixes <rdar://problem/12961769>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171607 91177308-0d34-0410-b5e6-96231b3b80d8