This adds basic support for eliding copy constructors. In order to make
this possible, it also adds support for returning structures. This
support does not include setting an NRVO flag when the class whose copy
is being elided has a non-trivial destructor.

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

This adds the support for implicit VTT arguments in constructors.

This adds the implementation of aggEmitFinalDestCopy for the case where
the destination value is not ignored. This requires adding the cir.copy
operation and associated interface code.

This fills in the missing pieces to handle volatile loads and stores in
CIR.

This addresses https://github.com/llvm/llvm-project/issues/153280

This change adds support for the throw op without sub expression and
with noreturn


Issue #154992

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.

This change adds support for the division operation between two complex
types

Issue: #141365