[clang] Improve -Wdeclaration-after-statement

With 118f966b, Clang matches GCC's behaviour and allows enabling
-Wdeclaration-after-statement with C99 and later.

However, the check for mixing declarations and code is not a constant time
algorithm, and therefore should be guarded with Diags.isIgnored().

Furthermore, improve test coverage with: non-pedantic C89 with the
warning; C11 with the warning; and when using -Wall.

Finally, mention the changed behaviour in ReleaseNotes.rst.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D117232