Support FRU pattern with `[feature(capture_disjoint_fields)]`
In case of a functional record update syntax for creating a structure, `ExprUseVisitor` to only detect the precise use of some of the field in the `..x` part of the syntax. However, when we start building MIR, we
1. First, build the place for `x`
2. and then, add precise field projections so that only some parts of `x` end up getting read.
When `capture_disjoint_fields` is enabled, and FRU is used within a closure `x` won't be completely captured, and therefore the first step will fail. This PR updates `mir_build` to create a place builder in the first step and then create place from the builder only after applying the field projection.
Closes https://github.com/rust-lang/project-rfc-2229/issues/32
r? ``````@nikomatsakis``````
7fa991fb85