Fix most GCC test failures.

This patch fixes almost all currently failing tests when
using GCC ToT.

The specific changes are:

(A) Workaround gcc.gnu.org/PR83921 which rejects variables w/o initializers
in constexpr contexts -- even when the variable is an empty class. This
bug has been worked around at all callsites by adding an initializer.
Additionally a new test, constexpr_init.pass.cpp, has been added to
test that Clang doesn't suffer from these bugs.

(B) Fix streambuf.assign/swap.pass.cpp. This test was never actually
calling the swap method as intended. In fact, the swap function it
intended to call was ill-formed when instantiated. GCC diagnosed
this ill-formedness w/o needing an instantiation.

(C) size_delete11.pass.cpp was fixed by adding c++2a to the list of
unsupported dialects.

llvm-svn: 322810