[ASTMatchers][NFC] integerLiteral(): Mention negative integers in

documentation.

Trying to match integerLiteral(-1) will silently fail, because an numeric
literal is always positive.
- Update the documentation to explain how to match negative numeric
  literals.
- Add a unit test.

Differential Revision: https://reviews.llvm.org/D35196

llvm-svn: 307663