Issue Description:

When statically linking Swift programs for Windows, duplicate symbols are emitted, preventing the object files/libs from being linked without /force:multiple (which is only supported on `link.exe` and not in LLD. This can be worked around by e.g. patching lld's COFF backend to ignore multiple definitions.

error: duplicate symbol: $Ss14CountableRangeVySiGAByxGs23BidirectionalCollectionsWL in /main.swift.o and in swiftcore.lib(swift.o)

error: duplicate symbol: __swift_memcpy24_8 in main.swift.o and in swiftcore.lib(swift.o)

error: duplicate symbol: $Ss23_ContiguousArrayStorageCyypGML in swiftcore.lib(swift.o) and in swiftswiftononesupport.lib(swiftononesupport.o)

Some of these functions should likely be marked 'COMDAT', so that they are used only when the compiler doesn't emit the implementation.

Similarly, swift_rt functions are (likely incorrectly) being exported twice:

/usr/bin/lld-link: error: duplicate symbol: swift_rt_swift_retain in main.swift.o and in swiftcore.lib(swift.o)
{{ /usr/bin/lld-link: error: duplicate symbol: swift_rt_swift_release in main.swift.o and in swiftcore.lib(swift.o)}}

which may also be a bug; as per @compnerd:

"The `swift_rt_` functions should not be exported I believe, as they are meant to be the local trampolines."