Arm64EC entry/exit thunks, consolidated.
This combines the previously posted patches with some additional work I've done to more closely match MSVC output.
Most of the important logic here is implemented in AArch64Arm64ECCallLowering. The purpose of the AArch64Arm64ECCallLowering is to take "normal" IR we'd generate for other targets, and generate most of the Arm64EC-specific bits: generating thunks, mangling symbols, generating aliases, and generating the .hybmp$x table. This is all done late for a few reasons: to consolidate the logic as much as possible, and to ensure the IR exposed to optimization passes doesn't contain complex arm64ec-specific constructs.
The other changes are supporting changes, to handle the new constructs generated by that pass.
There's a global llvm.arm64ec.symbolmap representing the .hybmp$x entries for the thunks. This gets handled directly by the AsmPrinter because it needs symbol indexes that aren't available before that.
There are two new calling conventions used to represent calls to and from thunks: ARM64EC_Thunk_X64 and ARM64EC_Thunk_Native. There are a few changes to handle the associated exception-handling info, SEH_SaveAnyRegQP and SEH_SaveAnyRegQPX.
I've intentionally left out handling for structs with small non-power-of-two sizes, because that's easily separated out. The rest of my current work is here. I squashed my current patches because they were split in ways that didn't really make sense. Maybe I could split out some bits, but it's hard to meaningfully test most of the parts independently.
I'm planning to write more IR tests for test coverage.