Fix crash in redundant-void-arg check.

Summary:
When applying this check to the unit tests, it would hit an assertion:
llvm/tools/clang/lib/Lex/Lexer.cpp:1056: clang::SourceLocation clang::Lexer::getSourceLocation(const char*, unsigned int) const: Assertion `PP && "This doesn't work on raw lexers"' failed.

Reviewers: klimek, LegalizeAdulthood, alexfh

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D14204

llvm-svn: 251792