Add pragma(musttail)

[MrSmith33]

Discussed in https://github.com/ldc-developers/ldc/discussions/4355

Open questions:

• pragma name. Is LDC_musttail what we want? Or we should use musttail?
• Need to add tests.

[kinke]

Or we should use musttail?

I like that better - I assume GDC could implement it too.

[kinke]

Wrt. a testcase, see e.g. https://github.com/ldc-developers/ldc/blob/master/tests/codegen/array_alloc_gh3041.d for a simple example checking the generated IR. The stuff in {{…}} is a regex. See https://llvm.org/docs/CommandGuide/FileCheck.html for the full reference if needed.

[JohanEngelen]

@ibuclaw Can GDC implement this too (or does it already have it?) ?

[thewilsonator]

I like that better - I assume GDC could implement it too.

cc @ibuclaw