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.
587af91045