[GR-2798] Rbdiag recognizes fallback specializations.

ci.hocon

...c/com/oracle/truffle/r/nodes/builtin/RBuiltinPackage.java

.../com/oracle/truffle/r/nodes/builtin/base/BasePackage.java

....test/src/com/oracle/truffle/r/nodes/casts/CastUtils.java

....nodes.test/src/com/oracle/truffle/r/nodes/casts/Not.java

...om/oracle/truffle/r/nodes/casts/TypeAndInstanceCheck.java

.../oracle/truffle/r/nodes/casts/UpperBoundsConjunction.java

...e/truffle/r/nodes/castsTests/ResultTypesAnalyserTest.java

.../src/com/oracle/truffle/r/nodes/test/ChimneySweeping.java

.../com/oracle/truffle/r/nodes/test/RBuiltinDiagnostics.java

...c/com/oracle/truffle/r/nodes/builtin/RBuiltinFactory.java

...oracle/truffle/r/runtime/builtins/RBuiltinDescriptor.java

...st/src/com/oracle/truffle/r/test/tools/RBuiltinCheck.java