cranelift-codegen: Rename abi::Caller to abi::CallSite

[fitzgen]

Will be less confusing since each call site has both a caller and a callee.

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[jameysharp]

Registering a drive-by vote in favor of this idea, but I'd still like @cfallin's review.

[cfallin]

This seems fine to me; @fitzgen and I talked it over and tried to brainstorm better names a bit yesterday and this is I think the clearest option.

Just to document a little history here: Caller (née ABICaller) was previously ABICall and Callee (née ABICallee) was previously ABIBody; the last rename happened in #2142 after some review comments; specifically here was where caller/callee were suggested, because "call" was confusing. Perhaps "callsite" is a bit better / more explicit though.

[alexcrichton]

Oh merge queue is configured for merge instead of squash right now so I'm going to bump this out while pat lands #6391