[C2x] Implement support for empty brace initialization (WG14 N2900 and WG14 N3011)

This implements support for allowing {} to consistently zero initialize
objects. We already supported most of this work as a GNU extension, but
the C2x feature goes beyond what the GNU extension allowed.

The changes in this patch are:

* Removed the -Wgnu-empty-initializer warning group. The extension is
  now a C2x extension warning instead. Note that use of
  `-Wno-gnu-empty-initializer seems` to be quite low in the wild
(https://sourcegraph.com/search?q=context%3Aglobal+-file%3A.*test.*+%22-Wno-gnu-empty-initializer%22&patternType=standard&sm=1&groupBy=repo
  which currently only gives 8 hits total), so this is not expected to
  be an overly disruptive change. But I'm adding the clang vendors
  review group just in case this expectation is wrong.
* Reworded the diagnostic wording to be about a C2x extension, added a
  pre-C2x compat warning.
* Allow {} to zero initialize a VLA

This functionality is exposed as an extension in all older C modes
(same as the GNU extension was), but does *not* allow the extension for
VLA initialization in C++ due to concern about handling non-trivially
constructible types.

Differential Revision: https://reviews.llvm.org/D147349