cmTarget: Move sanity checks and computed property access to callers

The GetProperty method is now just accessing contained data, meaning it
can be implemented in cmState.

Remove the cmMakefile context from the signature as a result and remove
the overload with the same signature.

Add a GetComputedProperty to cmTarget so that templates can be properly
instantiated.  Otherwise the Commands would need to be able to reach the
specializations which are currently in cmTarget.cxx.

As a side-effect, the CMP0026 warning now gives a backtrace to the
target when issued from a generator expression.
1 job for extract-computed-target-properties
Status Job ID Name Coverage
  External
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