[preprocessor] Fix assertion hit when 'SingleFileParseMode' option is enabled...

[preprocessor] Fix assertion hit when 'SingleFileParseMode' option is enabled and #if with an undefined identifier and without #else

'HandleEndifDirective' asserts that 'WasSkipping' is false, so switch to using 'FoundNonSkip' as the hint for 'SingleFileParseMode' to keep going with parsing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305940 91177308-0d34-0410-b5e6-96231b3b80d8