Avoid recursions when the parser finds out that it has too many brackets.

BalancedDelimiterTracker::diagnoseOverflow calls P.SkipUntil, and before this
patch P.SkipUnti is recursive, causing problems on systems with small stacks.
This patch fixes it by making P.SkipUnti non recursive when just looking for
eof.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187097 91177308-0d34-0410-b5e6-96231b3b80d8