More precisely recognize ObjC method declarations.

Otherwise, +/- and the beginning of constants can be recognized
incorrectly.

Before:  #define A - 1
After:   #define A -1

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177725 91177308-0d34-0410-b5e6-96231b3b80d8