With 118f966b46cf, 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.