Set lambda linkage to LinkOnceODR (was Internal)

[JohanEngelen]

This reverts #4415. The mangling of lambdas has been fixed (and is now stable and cannot clash with other modules anymore), so there is no longer a need for internal linkage. This allows merging of identical lambdas again (multiple instantiations).

This also fixes an instantiation bug, where the lambda's emission is sometimes culled because the frontend knows it is already instantiated in another module, but because of internal linkage it cannot resolve the symbol during linking (thus resulting in a missing symbol during linking).

[kinke]

Should be fine if you trust the stable mangles. ;) - Modulo test adaptations.

This also fixes an instantiation bug, where the lambda's emission is sometimes culled because the frontend knows it is already instantiated in another module

That sounds like an interesting bug on our side though, as emitting function/delegate literals into each referencing object file is our responsibility (not added to Module members).

[JohanEngelen]

@ljmf00 Perhaps you can dustmite the libwtracer.a build, to create a testcase? I have not been able to create a testcase by some ideas that I had (imported modules, ...)
Details: compile with LDC v1.38.0-weka9 (nota bene, -weka10 has the issue fixed; or with LDC v.1.38.0 vanilla), libwtracer.a contains a reference (declare, does not define) to symbol _D3std4conv__T6toImplTiTkZQmFkZ20__dgliteral_L561_C17MFNaNbNfZC6object9Throwable. Phobos binary does not export that symbol with LDC v1.38.0-weka9 (because internal linkage), so it should have been emitted into libwtracer.a.