CMake -GXcode would otherwise offer to create one scheme for each
target, which ends up being a lot. For now, limit the default to the
`check-*` LIT targets, plus `ALL_BUILD` and `install`.
For targets that aren't in the default list, we now have a configuration
variable to promote an extra list of targets into schemes, for example
`-DLLVM_XCODE_EXTRA_TARGET_SCHEMES="TargetParserTests;SupportTests"` to
add schemes for `TargetParserTests` and `SupportTests` respectively.
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