[clang] [clang] Implement constexpr heap allocation alignments (PR #174549)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I wanted to run this against the clang test suite but I can't merge it on top of main :thinking:  Can you rebase this? (and doesn't that need to happen anyway in the end? I'm confused)

https://github.com/llvm/llvm-project/pull/174549