Allow line numbers on -verify directives.

// expected-warning@10 {{some text}}

The line number may be absolute (as above), or relative to the current
line by prefixing the number with either '+' or '-'.

Patch by Andy Gibbs!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159978 91177308-0d34-0410-b5e6-96231b3b80d8