[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307663 91177308-0d34-0410-b5e6-96231b3b80d8