Fix test on Windows

'not' on Windows tries and fails to convert the argument to UTF-16 and back for
some reason:

  Error: Unable to convert command-line to UTF-16

Let's try replacing it with !.

(There was no obvious way to do these with FileCheck)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195907 91177308-0d34-0410-b5e6-96231b3b80d8
InclusionRewriter: Avoid duplicated BOM check

The lexer already knows its position in the file, so use that instead of
guessing it might be 3.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195910 91177308-0d34-0410-b5e6-96231b3b80d8