This adds the code needed to handle default initialization for fields of
various types.

This adds support for handling delegating constructors with VTT
arguments.

This adds the support for implicit VTT arguments in constructors.

This adds support for declaring a class with a virtual base class and
initializing the vptr in the constructor. This does not yet handle
constructors that require a virtual table table (VTT) implicit argument.

This reintroduces `Type.h`, having earlier been renamed to `TypeBase.h`,
as a redirection to `TypeBase.h`, and redirects most users to include
the former instead.

This is a preparatory patch for being able to provide inline definitions
for `Type` methods which would otherwise cause a circular dependency
with `Decl{,CXX}.h`.

Doing these operations into their own NFC patch helps the git rename
detection logic work, preserving the history.

This patch makes clang just a little slower to build (~0.17%), just
because it makes more code indirectly include `DeclCXX.h`.

This is a preparatory patch, to be able to provide inline definitions
for `Type` functions which depend on `Decl{,CXX}.h`. As the latter also
depends on `Type.h`, this would not be possible without some
reorganizing.

Splitting this rename into its own patch allows git to track this as a
rename, and preserve all git history, and not force any code
reformatting.

A later NFC patch will reintroduce `Type.h` as redirection to
`TypeBase.h`, rewriting most places back to directly including `Type.h`
instead of `TypeBase.h`, leaving only a handful of places where this is
necessary.

Then yet a later patch will exploit this by making more stuff inline.

This adds support for initializing the vptr members in a class that
requires multiple vtables because of multiple inheritence. This still
does not handle virtual bases.

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 change adds support for calling virtual functions. This includes
adding the cir.vtable.get_virtual_fn_addr operation to lookup the
address of the function being called from an object's vtable.

This adds support for the cir.vtable.get_vptr operation and uses it to
initialize the vptr member during constructors of dynamic classes.