[Bugpoint] Close error log in ProcessFailure. Bugpoint had been failing to close

[Bugpoint] Close error log in ProcessFailure. Bugpoint had been failing to close
this, and in some circumstances (e.g. reducing particularly large test-cases)
this was causing bugpoint to be killed for hitting open file-handle limits.

No test case: I was only able to trigger this with test cases taking upwards of
10 mins to run.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219244 91177308-0d34-0410-b5e6-96231b3b80d8