[SR-4349] Lock down the function signature lowering scheme

[bob-wilson]

Previous ID	SR-4349
Radar	rdar://problem/31408027
Original Reporter	@bob-wilson
Type	Task

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Task, AffectsABI | |Assignee | None | |Priority | Medium | md5: f7713ba5019cd1e466fc98e9d247f548

Issue Description:

From the ABI Stability manifesto:

Function signature lowering is the mapping of a function's source-language type, which includes formal parameters and results, all the way down to a physical convention, which dictates what values are stored in what registers and what values to pass on the stack.

ABI stability requires nailing down and fully specifying this algorithm so that future Swift versions can lower Swift types to the same physical call signature as prior Swift versions

[bob-wilson]

@swift-ci create

[bob-wilson]

We do not anticipate making any changes here, so the only thing left to do here is to document the existing behavior.