[driver][darwin] Pass -platform_version flag to the linker instead of the...

[driver][darwin] Pass -platform_version flag to the linker instead of the -<platform>_version_min flag

In Xcode 11, ld added a new flag called -platform_version that can be used instead of the old -<platform>_version_min flags.
The new flag allows Clang to pass the SDK version from the driver to the linker.
This patch adopts the new -platform_version flag in Clang, and starts using it by default,
unless a linker version < 520 is passed to the driver.

Differential Revision: https://reviews.llvm.org/D71579