The InstrProf headers are very expensive. Avoid including them in all of
CodeGen/ by moving the CodeGenPGO member behind a unqiue_ptr.

This reduces clang build time by 0.8%.

When records contain fields with pointer authentication, even simple
copies can require
additional work be performed. This patch contains the core functionality
required to
handle user defined structs, as well as the implicitly constructed
structs for blocks, etc.

Co-authored-by: Ahmed Bougacha
Co-authored-by: Akira Hatanaka
Co-authored-by: John Mccall

Follow-up to https://github.com/llvm/llvm-project/issues/123569

When building on Windows, dealing with the BlocksRuntime is slightly
more complicated. As we are not guaranteed a formward declaration for
the blocks runtime ABI symbols, we may generate the declarations for
them. In order to properly link against the well-known types, we always
annotated them as `__declspec(dllimport)`. This would require the
dynamic linking of the blocks runtime under all conditions. However,
this is the only the only possible way to us the library. We may be
building a fully sealed (static) executable. In such a case, the well
known symbols should not be marked as `dllimport` as they are assumed to
be statically available with the static linking to the BlocksRuntime.

Introduce a new driver/cc1 option `-static-libclosure` which mirrors the
myriad of similar options (`-static-libgcc`, `-static-libstdc++`,
-static-libsan`, etc).

Identified with misc-include-cleaner.

Correctly computing the LLVM types/attributes is complicated in general,
so add a variant which does that for you.

It is simple to create the struct body up front, now that we have
transitioned to opaque pointers.