Start improving diagnostics that relate to subcharacters of string literals.

First step, handle diagnostics in StringLiteral's that are due to token pasting.

For example, we now handle:
  id str2 = @"foo" 
            "bar"
           @"baz"
           " b\0larg";  // expected-warning {{literal contains NUL character}}

Correctly:

test/SemaObjC/exprs.m:17:15: warning: CFString literal contains NUL character
           " b\0larg";  // expected-warning {{literal contains NUL character}}
           ~~~^~~~~~~

There are several other related issues still to be done.

llvm-svn: 64924