cmTarget: Allow transitive evaluation of SOURCES property.

Extend the cmGeneratorExpressionDAGChecker with an interface
returning the name of the top target.  Use that to determine
when there is a DAG violation, as required by the RunCMake.Languages
tests.

Help/manual/cmake-properties.7.rst

@@ -152,6 +152,7 @@ Properties on Targets
    /prop_tgt/INTERFACE_INCLUDE_DIRECTORIES
    /prop_tgt/INTERFACE_LINK_LIBRARIES
    /prop_tgt/INTERFACE_POSITION_INDEPENDENT_CODE
+   /prop_tgt/INTERFACE_SOURCES
    /prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES
    /prop_tgt/INTERPROCEDURAL_OPTIMIZATION_CONFIG
    /prop_tgt/INTERPROCEDURAL_OPTIMIZATION

Help/prop_tgt/INTERFACE_SOURCES.rst

@@ -0,0 +1,15 @@
+INTERFACE_SOURCES
+-----------------
+
+List of interface sources to pass to the compiler.
+
+Targets may populate this property to publish the sources
+for consuming targets to compile.  Consuming
+targets can add entries to their own :prop_tgt:`SOURCES` property
+such as ``$<TARGET_PROPERTY:foo,INTERFACE_SOURCES>`` to use the
+sources specified in the interface of ``foo``.
+
+Contents of ``INTERFACE_SOURCES`` may use "generator expressions"
+with the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
+manual for available expressions.  See the :manual:`cmake-buildsystem(7)`
+manual for more on defining buildsystem properties.

Help/release/dev/target-INTERFACE_SOURCES.rst

@@ -0,0 +1,5 @@
+target-INTERFACE_SOURCES
+------------------------
+
+* A new :prop_tgt:`INTERFACE_SOURCES` target property was introduced. This is
+  consumed by dependent targets, which compile and link the listed sources.