[lld-macho][nfc] Set test min version to 11.0

[int3]

Summary

The arm64-apple-macos triple is only valid for versions >= 11.0. (If
one passes arm64-apple-macos10.15 to llvm-mc, the output's min version is still
11.0). In order to write tests easily for both target archs, let's up the
default min version in our tests.

[thakis]

I don't have a problem with the change, but I also don't understand the motivation. llc (and clang) transparently setting the os to 11.0 for arm is a feature (there are no arm macs running 10.15). It makes it _easier_ to write your build files (want to target 10.14? set mmacosx-version-min=10.14, and it'll automatically do the right thing in the arm part of your build).

lld tests that want to test version handling want to use %no-arg-lld anyways so we don't have to change everything when we change this default.

So I don't get the benefit behind this change. (But I also don't see a problem with it, so feel free to land if it's useful in some way I miss!)

[int3]

Ah I want this for my eh-frame.s test in D124561: [lld-macho] Support EH frames under arm64 in order that I can use %lld -arch arm64 rather than %no-arg-lld. (Since the macOS TBDs support both x86_64 and arm64, there's no need to specify a different -platform_version or -syslibroot.)

lld/test/MachO/lit.local.cfg

lld_watchos = ('ld64.lld -arch arm64_32 -platform_version watchos 7.0 8.0 -syslibroot ' +
     os.path.join(config.test_source_root, "MachO", "Inputs", "WatchOS.sdk"))
 config.substitutions.append(('%lld-watchos', lld_watchos + ' -fatal_warnings'))
 config.substitutions.append(('%no-fatal-warnings-lld-watchos', lld_watchos))
 
 config.substitutions.append(('%no-arg-lld', 'ld64.lld'))
 
 # Since most of our tests are written around x86_64, we give this platform the
 # shortest substitution of "%lld".
-lld = ('ld64.lld -arch x86_64 -platform_version macos 10.15 11.0 -syslibroot ' +
+lld = ('ld64.lld -arch x86_64 -platform_version macos 11.0 11.0 -syslibroot ' +
     os.path.join(config.test_source_root, "MachO", "Inputs", "MacOSX.sdk"))
 config.substitutions.append(('%lld', lld + ' -fatal_warnings'))
 config.substitutions.append(('%no-fatal-warnings-lld', lld))