Generate CodeView by deafult for Windows

[lanza]

llvm, lldb and swift combine to offer better support for PDBs on
Windows than it does DWARF. Default to generating PDBs on Windows.

[lanza]

After #22624 goes in this would be the most sane default.

[compnerd]

Please add a driver level test to ensure that we do this.

[compnerd]

CC: @dcci

[lanza]

@compnerd This fails to build on Windows due to the pdb gen bug you hit. WIP

[CodaFi]

Let's see what happens

@swift-ci please smoke test

[CodaFi]

@swift-ci please test Windows platform

[compnerd]

welp, thats going to destroy the test suite on Windows; I think that we should ensure that those work before merging this

[dcci]

Agree. Also it feels like a bug to me that Windows failing doesn't prevent the PR from being merged.

[CodaFi]

@lanza I think we can merge this if you XFAIL or fix these tests:

    Swift(windows-x86_64) :: IRGen/condfail_message.swift
    Swift(windows-x86_64) :: DebugInfo/variables.swift
    Swift(windows-x86_64) :: DebugInfo/tuple.swift
    Swift(windows-x86_64) :: DebugInfo/top_level_code.swift
    Swift(windows-x86_64) :: DebugInfo/shadowcopy-linetable.swift
    Swift(windows-x86_64) :: DebugInfo/scopes.swift
    Swift(windows-x86_64) :: DebugInfo/return.swift
    Swift(windows-x86_64) :: DebugInfo/prologue.swift
    Swift(windows-x86_64) :: DebugInfo/patternmatching.swift
    Swift(windows-x86_64) :: DebugInfo/linetable.swift
    Swift(windows-x86_64) :: DebugInfo/line-directive.swift
    Swift(windows-x86_64) :: DebugInfo/letstring.swift
    Swift(windows-x86_64) :: DebugInfo/iteration.swift
    Swift(windows-x86_64) :: DebugInfo/inlinedAt.swift
    Swift(windows-x86_64) :: DebugInfo/inlinescopes.swift
    Swift(windows-x86_64) :: DebugInfo/implicitreturn.swift
    Swift(windows-x86_64) :: DebugInfo/gsil.swift
    Swift(windows-x86_64) :: DebugInfo/foreach.swift
    Swift(windows-x86_64) :: DebugInfo/fnptr.swift
    Swift(windows-x86_64) :: DebugInfo/enum.swift
    Swift(windows-x86_64) :: DebugInfo/columns.swift
    Swift(windows-x86_64) :: DebugInfo/closure-args.swift
    Swift(windows-x86_64) :: DebugInfo/basic.swift
    Swift(windows-x86_64) :: DebugInfo/bool.swift
    Swift(windows-x86_64) :: DebugInfo/any.swift
    Swift(windows-x86_64) :: DebugInfo/alloca-init.swift
    Swift(windows-x86_64) :: DebugInfo/NestedTypes.swift
    Swift(windows-x86_64) :: DebugInfo/Imports.swift
    Swift(windows-x86_64) :: DebugInfo/ImportsStdlib.swift
    Swift(windows-x86_64) :: DebugInfo/EagerTypeMetadata.swift

[dcci]

I wouldn't really XFAIL these tests. It means that there are potentially big holes in CodeView Windows support and merging this patch would lead to false expectations of what works and what doesn't (or maybe tests just need to be updated and everything works fine, in which case, well, we should do it sooner rather than later).

[CodaFi]

@lanza I'm going to close this out since it seems like this needs to be handled incrementally to avoid disrupting the Windows build.