[ms] Fix mangling of string literals used to initialize arrays larger or smaller than the literal

A Chromium developer reported a bug which turned out to be a mangling
collision between these two literals:

  char s[] = "foo";
  char t[32] = "foo";

They may look the same, but for the initialization of t we will (under
some circumstances) use a literal that's extended with zeros, and
both the length and those zeros should be accounted for by the mangling.

This actually makes the mangling code simpler: where it previously had
special logic for null terminators, which are not part of the
StringLiteral, that is now covered by the general algorithm.

(The problem was reported at https://crbug.com/857442)

Differential Revision: https://reviews.llvm.org/D48928

llvm-svn: 336415