This patch adds basic support for constant record initializer list
expressions. There's a couple of limitations:

* No zero initialized padding bytes in C mode
* No bitfields
* No designated initializer lists
* Record alignments are not calculated, yet
* ILEs of derived records don't work, yet
* The constant attribute is not propagated to the backend, resulting in
non-constants being emitted in the LLVM IR

This adds the support for implicit VTT arguments in constructors.

And make use of those.

These changes are split from prior PR #155028, in order to decrease the
size of that PR and facilitate review.

This changes a bunch of places which use getAs<TagType>, including
derived types, just to obtain the tag definition.

This is preparation for #155028, offloading all the changes that PR used
to introduce which don't depend on any new helpers.

The getCIRSourceLanguage wasn't returning a value if the source language
was anything other than C or C++. This change updates that function to
return a std::optional value and only adds the source language attribute
if one was returned.

This adds a simplified version of the code to emit vtables. It does not
yet handle RTTI or cases that require multiple vtables.

This patch upstreams `SourceLangAttr` and its CodeGen logic in the CGM,
which encodes the source language in CIR.

This patch adds the constant attribute to cir.global, the appropriate
lowering to LLVM constant and updates the tests.

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Co-authored-by: Andy Kaylor <akaylor@nvidia.com>

This patch handles both implicit and explicit template instantiations of
template class static member variables.