[NFC][Clang] Fix static code analyzer concern about null value dereference

CurLexer is dereferenced and should not be null in clang::Preprocessor::SkipExcludedConditionalBlock(clang::SourceLocation, clang::SourceLocation, bool, bool, clang::SourceLocation)

This patch adds an assert for NULL value check of pointer CurLexer and splits up all predicates so that, when/if a failure occurs, we'll be able to tell which predicate failed.

Reviewed By: tahonermann

Differential Revision: https://reviews.llvm.org/D158293