cc1as: fix a potential leak and unremoved output file in error conditions

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209935 91177308-0d34-0410-b5e6-96231b3b80d8
cc1as: invert return bool to indicate failure

This simplifies code flow and matches the convention used in surrounding code.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209936 91177308-0d34-0410-b5e6-96231b3b80d8