Xcode: Fix detection of default language standard when given -std= flags

If one uses `CFLAGS='-std=...'` or `CXXFLAGS='-std=...'` then the given
`-std=` flag(s) will always be used.  That effectively changes the
compiler default standard level and extension settings.  Fix the
Xcode generator's compiler id logic to preserve any `-std=` flag
so that the proper defaults are detected.

This problem was exposed by commit 4a0485be7f (cmStandardLevelResolver:
Avoid unnecessary flags, fix unset level logic, 2021-05-29), which
changed the logic to not pass any `-std=` flag if the standard level
and extension settings requested by the project match the default
(`stdIt <= defaultStdIt` became `stdIt < defaultStdIt`).  The new
logic assumes the detected default standard matches what will actually
happen when the project is generated.