[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

Author: smanna12

clang/lib/Lex/PPDirectives.cpp

@@ -491,7 +491,9 @@ void Preprocessor::SkipExcludedConditionalBlock(SourceLocation HashTokenLoc,
   llvm::SaveAndRestore SARSkipping(SkippingExcludedConditionalBlock, true);
 
   ++NumSkipped;
-  assert(!CurTokenLexer && CurPPLexer && "Lexing a macro, not a file?");
+  assert(!CurTokenLexer && "Conditional PP block cannot appear in a macro!");
+  assert(CurPPLexer && "Conditional PP block must be in a file!");
+  assert(CurLexer && "Conditional PP block but no current lexer set!");
 
   if (PreambleConditionalStack.reachedEOFWhileSkipping())
     PreambleConditionalStack.clearSkipInfo();