Fix errors and warnings building swift/IRGen on Windows using MSVC

[hughbe]

• Uncovered control path errors
• 32bit/64 bit conversion warnings
• Unused result warnings
• Syntax error "template" errors - let me know if I should remove the code that runs for clang currently - it seems like a misplaced template, but I'm no expert

And, don't worry. We're not quite halfway through my PR attack, but for now I'm gonna stop and ease your workload.

[hughbe]

Thanks @compnerd

• I pushed the #pragma clang diagnostic ignored "-Winconsistent-missing-override" to CMake
• I removed the "Workaround MSVC warning comments" and updated the content of the llvm_unreachables
• I sinked the cast and renamed the variable

The variable is used so why is the cast to void needed?

MSVC doesn't think so - I think its because the first check if SWIFT_RT_USE_RegisterPreservingCC which it identifies as a constant

[jrose-apple]

I think we actually just want to fix these in most cases. Those particular files really are different from the others.

[jrose-apple]

…and yes, that does deserve a comment in those files.

[hughbe]

I reverted this commit, removed the MSVC checks (as clang-cl needs them but defined MSC_VER). We also now ignore unknown pragmas so this is pointless.
I also added a TODO to the pragmas that I shall try to address once I'm not so busy. Thanks

[jckarter]

The template shouldn't be required by Clang, either, since no explicit template arguments are being given. We should be able to unconditionally remove it from both of these places.

[hughbe]

Awesome, fixed

[jrose-apple]

No, I tried once. It really doesn't make sense with the way these files are using overrides—they're coming from templates. (Or macros, I can't remember.)

[hughbe]

Always a reason. I reverted the TODOs. There are no changes to the pragmas in this PR

[jrose-apple]

@jckarter, does this seem all right to you now?

@swift-ci Please smoke test

[jckarter]

LGTM.