rustc: Fix mixing crates with different share_generics

[alexcrichton]

This commit addresses #64319 by removing the dylib crate type from the
list of crate type that exports generic symbols. The bug in #64319
arises because a dylib crate type was trying to export a symbol in an
uptream crate but it miscalculated the symbol name of the uptream
symbol. This isn't really necessary, though, since dylib crates aren't
that heavily used, so we can just conservatively say that the dylib
crate type never exports generic symbols, forcibly removing them from
the exported symbol lists if were to otherwise find them.

The fix here happens in two places:

• First is in the local_crate_exports_generics method, indicating that
  it's now false for the Dylib crate type. Only rlibs actually
  export generics at this point.

• Next is when we load exported symbols from upstream crate. If, for our
  compilation session, the crate may be included from a dynamic library,
  then its generic symbols are removed. When the crate was linked into a
  dynamic library its symbols weren't exported, so we can't consider
  them a candidate to link against.

Overally this should avoid situations where we incorrectly calculate the
upstream symbol names in the face of differnet share_generics options,
ultimately...

Closes #64319