MS ABI: Add tests, other cleanups for r204562

This commit cleans up a few accidents:
- Do not rely on the order in which StringLiteral lays out bytes.
- Use a more efficient mechanism for handling so-called
  "special-mappings" when mangling string literals.
- There is no need to allocate a copy of the mangled name.
- Add the test written for r204562.

Thanks to Richard Smith for pointing these out!
CodeGen: Remove unneeded directives

Use two check-prefix patterns per FileCheck invocation for these tests,
this cleanly removes redundant CHECK directives.

Thanks to Richard Smith for the idea!
Driver: Fix a typo in a command line description

Thanks to Richard Smith for catching this!