[lldb/Platform] Skip very slow xcrun queries for simulator platforms, NFC

GetXcodeSDK() consistently takes over 1 second to complete if the
queried SDK is missing, because `xcrun` doesn't cache negative lookups.

Because there are multiple simulator platforms, this can add 4+ seconds
to `lldb -b some_object_file.o`.

To work around this, skip the call to GetXcodeSDK() when setting up
simulator platforms if the specified arch doesn't have what looks like a
simulator triple.

Some other ways to fix this:
- Fix caching in xcrun (rdar://74882205)
- Test for arch compat before calling SomePlatform::CreateInstance() (much
  larger change)

Differential Revision: https://reviews.llvm.org/D98272