Add basic HS/DS implementation.

Add basic HS/DS implementation.

This obsoletes WIP PR #704, which was built on the pre entry point wrapping master.  New version
here uses entry point wrapping.

This is a limited implementation of tessellation shaders.  In particular, the following are not functional,
and will be added as separate stages to reduce the size of each PR.

* patchconstantfunctions accepting per-control-point input values, such as
  const OutputPatch <hs_out_t, 3> cpv are not implemented.

* patchconstantfunctions whose signature requires an aggregate input type such as
  a structure containing builtin variables.  Code to synthesize such calls is not
  yet present.

These restrictions will be relaxed as soon as possible.  Simple cases can compile now: see for example
Test/hulsl.hull.1.tesc - e.g, writing to inner and outer tessellation factors.

PCF invocation is synthesized as an entry point epilogue protected behind a barrier and a test on
invocation ID == 0.  If there is an existing invocation ID variable it will be used, otherwise one is
added to the linkage.  The PCF and the shader EP interfaces are unioned and builtins appearing in
the PCF but not the EP are also added to the linkage and synthesized as shader inputs.
Parameter matching to (eventually arbitrary) PCF signatures is by builtin variable type.  Any user
variables in the PCF signature will result in an error.  Overloaded PCF functions will also result in
an error.

[domain()], [partitioning()], [outputtopology()], [outputcontrolpoints()], and [patchconstantfunction()]
attributes to the shader entry point are in place, with the exception of the Pow2 partitioning mode.