Add tests for preallocated + musttail

Summary:
Follow-up to https://reviews.llvm.org/D80581.
Turns out the codegen part already worked, so only needed to add tests.
I manually verified that in these tests the generated code for inalloca
and preallocated were identical.

Reviewers: efriedma, hans

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80742