Enable MIR inlining

[cjgillot]

Continuation of #82280 by @wesleywiser.

#82280 has shown nice compile time wins could be obtained by enabling MIR inlining.
Most of the issues in #81567 are now fixed,
except the interaction with polymorphization which is worked around specifically.

I believe we can proceed with enabling MIR inlining in the near future
(preferably just after beta branching, in case we discover new issues).

Steps before merging:

• figure out the interaction with polymorphization;
• figure out how miri should deal with extern types;
• silence the extra arithmetic overflow warnings;
• remove the codegen fulfilment ICE;
• remove the type normalization ICEs while compiling nalgebra;
• tweak the inlining threshold.