Rollup merge of #117475 - nnethercote:rm-create_session, r=oli-obk

Inline and remove `create_session`.

Currently the parts of session initialization that happen within `rustc_interface` are split between `run_compiler` and `create_session`. This split isn't necessary and obscures what's happening.

This commit merges the two functions. I think a single longer function is much clearer than splitting this code across two functions in different modules, especially when `create_session` has 13 parameters, and is misnamed (it also creates the codegen backend). The net result is 43 fewer lines of code.

r? ``@oli-obk``

Author: matthiaskrgr