`rustc_mir::pretty` refactoring: break `fn write_fn_intro` into two routines.
`rustc_mir::pretty` refactoring: break `fn write_fn_intro` into two routines.

(The crucial thing these changes are working toward (but are not yet
in this commit) is a way to pretty-print MIR without having the
`NodeId` for that MIR in hand.)

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