Added conditional guard 'if (CurLexer)' when using SetCommentRetentionState().

This is because the PTHLexer will not support this method. Performance testing
on preprocessing Cocoa.h shows that this results in a negligible performance
difference (less than 1%).

I tried making Lexer::SetCommentRetentionState() an out-of-line function (a
precursor to making it a virtual function in PreprocessorLexer) and noticed a 1%
decrease in speed (it is called in a hot part of the Preprocessor).

llvm-svn: 59477