Mirror clang's link stage on Windows.

[dot-asm]

Fixes problem discussed in #811.

[ChrisDenton]

Hm, unsure about the implementation of this. Since we're forcing this library to be included, we need to be careful that it doesn't override anything the user has implemented themselves. clang does this using -defaultlib:oldnames.lib to give the library lower priority than non-default libs.

[dot-asm]

cargo:rustc-link-arg=-defaultib:oldnames then. Deeper dive into -vvv logs for a multi-file project revealed that it's also a wrong spot for the link arguments. [As well as --target duplication thing.]

[ChrisDenton]

This could really use a test to make sure it works and isn't accidentally broken in the future.

[ChrisDenton]

Also cc @thomcc as using rustc-link-arg would require bumping the MSRV to >=1.56 (October 2021).

[thomcc]

That MSRV bump doesn't bother me, but I would really like someone to double-check this works. My recollection is that there's some issue with using rustc-link-arg like this.

[dot-asm]

Also cc @thomcc as using rustc-link-arg would require bumping the MSRV to >=1.56 (October 2021).

Hmm, that would probably be undesirable... Two points in the context. One should keep in mind that the suggested modification affects only "fringe" use cases, an MSVC target with $CC set to clang, as opposed to clang-cl, which by itself is error-prone in the most general case. With this in mind, secondly, since the directive is ignored by an older version, then it wouldn't pose a problem for users who use the older version.

[dot-asm]

Putting together a test case was challenging because one has to mix object files generated by cl and clang in the same executable, with the former smuggling in oldnames.lib through the linker directive. -Zl prevents it from doing this and thus shifts the burden of linking in its entirety on rustc. You can convince yourself that the test is affirmative by commenting the line that adds -defaultlib:oldnames link flag and noting that the cc-test fails to link with "unresolved external symbol strdup" message. Well, this is provided that you have clang on the %PATH%. Just in case, this is the case on Github Actions, which can be confirmed by noting test msvcrt_here ... ok in the log files.

[dot-asm]

Re-based just in case.