test: adjust tests for Windows after the rebranch

[compnerd]

This corrects the name decoration and splits the windows and itanium decoration schemes. This allows us to un-XFAIL some of the interop tests on Windows while repairing the failures identified on rebranch.

[etcwilde]

@swift-ci please smoke test

[etcwilde]

Linux is still hitting the expected clang/DebugInfo crash.
Windows is down from 6 failures to 4 failures;

Failed Tests (4):
  Swift(windows-x86_64) :: DebugInfo/implicitreturn.swift
  Swift(windows-x86_64) :: Driver/Dependencies/only-skip-once.swift
  Swift(windows-x86_64) :: IRGen/c_globals.swift
  Swift(windows-x86_64) :: IRGen/framepointer.sil

This fixes

  Swift(windows-x86_64) :: Interop/Cxx/operators/member-inline-irgen.swift
  Swift(windows-x86_64) :: Interop/Cxx/operators/member-out-of-line-irgen.swift

This all makes sense given the changes. Recording for the sake of tracking.

macOS is somehow still building llvm after 3 hours, but I'd like to verify that it doesn't break these two tests before merging.

[etcwilde]

macOS failure

********************
Failed Tests (1):
  lldb-api :: lang/swift/async/stepping/step-in/TestSwiftStepInAsync.py

********************
Unexpectedly Passed Tests (1):
  lldb-api :: lang/swift/async/stepping/step-in/task-switch/TestSwiftTaskSwitch.py

[compnerd]

@swift-ci please test

[etcwilde]

All failures are known and expected. Merging.