Parse: Don't parse beyond the end of the synthetic default argument tok

Parse: Don't parse beyond the end of the synthetic default argument tok

Recovery from malformed lambda introducers would find us consuming the
synthetic default argument token, which is bad.  Instead, stop right
before that token.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225613 91177308-0d34-0410-b5e6-96231b3b80d8