[5.3][Function Builders] Teach diagnostics about function builder body result types...

[hborla]

...in order to properly diagnose requirement failures on a builder-transformed type.

This is mostly a cherry-pick of #33149, but working around not being able to anchor the constraint between the function result type and the transformed type on the function decl.

Reviewed by @xedin

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• Add a new locator path element FunctionBuilderBodyResult
• When adding the constraint to match the function builder transformed type with the body result type, add the FunctionBuilderBodyResult path element.
• When computing the affected decl for a RequirementFailure that has a locator pointing to a FunctionBuilderBodyResult, get the affected decl from the function result type
• Override getLoc in RequirementFailure to always produce the diagnostic at the function declaration when the fix is for a FunctionBuilderBodyResult, since the anchor can be nullptr

Resolves: rdar://problem/66247196

[hborla]

@swift-ci please test