debuginfo: Instruct MSVC linker to generate PDB file if debuginfo is enabled

[michaelwoerister]

This PR will enable RUSTC to generate PDB debuginfo files when targeting the MSVC toolchain. Mind that these are not full featured PDB files -- they just contain line tables, so you can get proper backtraces and step through your code, but variable values can't be inspected. We are just levering (LLVM's current support)[http://clang.llvm.org/docs/MSVCCompatibility.html] for creating Windows debuginfo. When LLVM's support gets better, we should benefit from that too without much effort.

I also wanted to include some kind of auto test with this PR but I could not get the rmake tests to work properly when targeting MSVC.

EDIT:
Closes #19533

[rust-highfive]

r? @arielb1

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[michaelwoerister]

Oops, trailing whitespace in there. I'll fix it asap.

[michaelwoerister]

OK, should be fixed now.

[alexcrichton]

Is there a downside to just always passing /DEBUG to the linker? For example if we're linking native libraries with debuginfo into a Rust library with not debuginfo, perhaps it'd be good to pass the flag through?

[retep998]

Passing /DEBUG disables /OPT by default, although if Rust explicitly specifies /OPT (which it does) that will take precedence.

[alexcrichton]

@bors: r+ 6c70127

Ah ok, sounds like we definitely shouldn't be passing by default then.

[michaelwoerister]

/DEBUG also implies /INCREMENTAL which leads to larger binaries because of additional padding.

[bors]

⌛️ Testing commit 6c70127 with merge 2533a85...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt