Add noalias/allocsize attributes to GC allocation runtime functions

[kubo39]

Mark the returned pointer of _d_allocmemory, _d_allocmemoryT and _d_newclass/_d_allocclass with noalias, and give _d_allocmemory an allocsize(0).
This helps alias analysis and enables llvm's objectsize-based optimizations.

[thewilsonator]

Looks good,

The following tests FAILED:
	1716 - druntime-test-exceptions-release (Failed)

not sure why though.

[JohanEngelen]

Are we sure this works when doing something like:

new
delete (GC collection)
new
delete (GC collection)
new
delete (GC collection)
new

where all news might actually return the same memory address. Will LLVM's optimizer make the right decisions?

A delete means that we have no references any more to the item, so in D code, the noalias must be correct because any new call will return a pointer that is indeed not aliasing any existing memory reference any more.
However, in the LLVM IR optimizer, this "temporal" connection might be gone after reordering of code. I did not think this through myself yet.

[kubo39]

Looks like it's because of SimplifyDRuntimeCalls.