Add detailed diagnostics for E0386.
Add detailed diagnostics for E0386.

This adds detailed diagnostics for E0386, 'cannot assign to data in an
immutable container'.

dwillmer/rust,dwillmer/rust,aidancully/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,dwillmer/rust,graydon/rust,graydon/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,dwillmer/rust