Linking with MSVC toolchain

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This is a rebased PR of flang-compiler/clang#37 by @albertziegenhagel

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This is not enough for building Fortran projects using cmake.

In UNIX, cmake would compile a test file and inspect the executable to get the fortran runtime libraries to link to.

In Windows, this is more complicated and clang would embed the linking options in the object files. This is done by adding llvm.linker.options

You can see this by doing,
clang-cl empty.c -c -Xclang -emit-llvm && cat empty.obj which gives me,

; ModuleID = 'empty.c'
source_filename = "empty.c"
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc19.11.0"

; Function Attrs: noinline nounwind optnone sspstrong uwtable
define i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  ret i32 1
}

attributes #0 = { noinline nounwind optnone sspstrong uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.linker.options = !{!0, !1}
!llvm.module.flags = !{!2, !3}
!llvm.ident = !{!4}

!0 = !{!"/DEFAULTLIB:libcmt.lib"}
!1 = !{!"/DEFAULTLIB:oldnames.lib"}
!2 = !{i32 1, !"wchar_size", i32 2}
!3 = !{i32 7, !"PIC Level", i32 2}
!4 = !{!"clang version 6.0.1 "}

How would you want to do this? In an old PR, what I did was pass them to flang2 like -linker /DEFAULTLIB:libcmt.lib -linker /DEFAULTLIB:oldnames.lib. Is this acceptable?

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See also the discussion at https://gitlab.kitware.com/cmake/cmake/issues/17384#note_337478

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Depends on flang-compiler/flang#729