Add an explanation of -Wobjc-literal-compare to the "Objective-C Literals" page

(per Jean-Daniel's suggestion to keep around an explanation of why
direct comparisons on ObjC literals are a bad idea)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159972 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed