[clang][Darwin] Simply deployment version assignment in the Driver

To be able to handle all of the ways the platform & deployment version can be represented in command line flags, the Darwin toolchain holds a type DarwinPlatform to help represent them. This patch simplifies the logic by:

reducing the amount of work done between string & version tuples conversions
renaming variables to reduce confusion about what target triple information is being manipulated.
allowing implicit transformation of macOS10.16 -> 11, there are other places in the compiler where this happens, and it was a bit confusing that the driver didn't do that for the cc1 call.
This is not a major refactor, but more simple & common tweaks across the file, in hopes to make it more readable.

Author: cyndyishida

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -206,6 +206,7 @@ def err_drv_cannot_open_randomize_layout_seed_file : Error<
   "cannot read randomize layout seed file '%0'">;
 def err_drv_invalid_version_number : Error<
   "invalid version number in '%0'">;
+def err_drv_missing_version_number : Error<"missing version number in '%0'">;
 def err_drv_kcfi_arity_unsupported_target : Error<
   "target '%0' is unsupported by -fsanitize-kcfi-arity">;
 def err_drv_no_linker_llvm_support : Error<
@@ -478,6 +479,9 @@ def warn_ignoring_ftabstop_value : Warning<
 def warn_drv_overriding_option : Warning<
   "overriding '%0' option with '%1'">,
   InGroup<DiagGroup<"overriding-option">>;
+def warn_drv_overriding_deployment_version
+    : Warning<"overriding deployment version from '%0' to '%1'">,
+      InGroup<DiagGroup<"overriding-deployment-version">>;
 def warn_drv_treating_input_as_cxx : Warning<
   "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">,
   InGroup<Deprecated>;