Resolve and complete record initialization

[bbannier]

Currently we do not resolve fields in record initializers, e.g.,

type X: record { field_name: string; };

global x: X = [ $field_name="" ];
#               ^ hover missing

it would also be nice if we would be able to provide completion possible field names. This requires us to be able to resolve the type of the value we are creating (either from a type on the LHS, or from the type of a function argument) from potentially incomplete code.