Continue parsing an expression list even after an error is encountered.

Otherwise, multiple errors such as having unknown identifiers for two
arguments won't be diagnosed properly (e.g. only the first one would
have a diagnostic message if typo correction fails even though both
would be diagnosed if typo correction suggests a replacement).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213003 91177308-0d34-0410-b5e6-96231b3b80d8