There's no reason to use -m${platform}-version-min as of clang-11/Xcode 11. Clang is now smart enough to parse -target and provide Apple's ld with the appropriate -platform_version argument string.
The standard library (and other Swift modules built by our CMake build system)
has been building module files with an architecture only (e.g., x86_64.swiftmodule)
rather than a proper module triple (x86_86-apple-macosx10.15,
x86_64-apple-ios13.0-simulator, etc.), unlike every other build
system. There are hacks in the compiler and other tools to cope with
this unnecessary build difference. Fix the module file names so we'll
be able to remove the hacks later.
Fixes rdar://problem/49071536.
