Start improving diagnostics that relate to subcharacters of string literals.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64924 91177308-0d34-0410-b5e6-96231b3b80d8