function body, map the result builder type into context.
This was already done for inferred result builder attributes; now,
the constraint system will map the builder type into context for all
result builder attributes applied to computed properties/functions.
always map type parameters into context.
For result builders applied to function/variable/subscript bodies, the builder
type will only be used for calls to `buildBlock` and friends inside the body,
for which we always want the contextual type of the builder, not the interface
type.
