Fix a theoretical bug when ParseCompoundStatement() returns StmtError.

ParseCompoundStatement() currently never returns StmtError, but if it did,
Sema would keep the __finally scope on its stack indefinitely.  Explicitly
add an error callback that clears it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231625 91177308-0d34-0410-b5e6-96231b3b80d8