Auto merge of #47209 - eddyb:ccx, r=nikomatsakis

rustc_trans: reorganize CrateContext and rename context types.

Firstly, the `{Shared,Local}CrateContext` hasn't been meaningful for a while now, and this PR resolves it by moving all their fields to `CrateContext` and removing redundant accessor methods.

Secondly, this PR contains the following mass-renames:
* `ccx: CrateContext` -> `cx: CodegenCx`
* `mircx: MirContext` -> `fx: FunctionCx`
* `bcx: Builder` -> `bx: Builder`

r? @nikomatsakis