This patch upstreams support for writing inline and out of line C++
destructor definitions. Calling a destructor implcitly or explicitly is
left for a future patch.

Because of that restriction complete destructors (D2 in Itanium
mangling) do not call into the base (D1) destructors yet but simply
behave like a base destructor. Deleting (D0) destructor support is not
part of this patch.

Destructor aliases aren't supported, either. Because of this compilation
with -mno-constructor-aliases may be required to avoid running into NYI
errors.

These decl types don't require any code generation, though when debug
info is implemented, we will need to add handling for that. Until then,
we just need to have a handler so they don't generate an NYI error.

This patch adds the `#cir.opt_info` attribute which holds module-level
optimization information.

This adds support for assignment operators, including implicit operator
definitions.

This adds the code to handle operator new expressions in ClangIR.

This change adds support for handling the -mconstructor-aliases option
in CIR. Aliases are not yet correctly lowered to LLVM IR. That will be
implemented in a future change.

This fixes a warning where a variable assigned in 'if' statement wasn't
referenced again.

This change adds support for function linkage and visibility and related
attributes. Most of the test changes are generalizations to allow
'dso_local' to be accepted where we aren't specifically testing for it.
Some tests based on CIR inputs have been updated to add 'private' to
function declarations where required by newly supported interfaces.

The dso-local.c test has been updated to add specific tests for
dso_local being set correctly, and a new test, func-linkage.cpp tests
other linkage settings.

This change sets `comdat` correctly in CIR, but it is not yet applied to
functions when lowering to LLVM IR. That will be handled in a later
change.

Upstream the code to handle member variable initialization in a
constructor. At this point only simple scalar values (including members
of anonymous unions) are handled.

This change upstreams the code to emit simple constructor defintions.