Add a VM to EmitC conversion and CModule Target (#2536)

Makes first progress on #1173:
* Defines `IREE_HAVE_EMITC_DIALECT` to enable conditional builds, which
  are CMake only so far
* Enables the EmitC dialect in iree-opt and iree-translate
* Adds a VMToEmitC conversion (test) pass
* Adds a C target to the VM

So far an empty file is written. In a follow up, we will implement
EmitC -> textual C, (re)using EmitC's CppTarget within the CModule target.

Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>

build_tools/cmake/iree_copts.cmake

diff --git a/build_tools/cmake/iree_copts.cmake b/build_tools/cmake/iree_copts.cmake
index c39db21..7563c1f 100644
--- a/build_tools/cmake/iree_copts.cmake
+++ b/build_tools/cmake/iree_copts.cmake
@@ -250,3 +250,15 @@
   ${PROJECT_BINARY_DIR}/build_tools/third_party/tensorflow
   ${PROJECT_BINARY_DIR}/build_tools/third_party/tensorflow/tensorflow/compiler/mlir/hlo/include/
 )
+
+#-------------------------------------------------------------------------------
+# Third party: mlir-emitc
+#-------------------------------------------------------------------------------
+
+if(IREE_ENABLE_EMITC)
+  list(APPEND IREE_COMMON_INCLUDE_DIRS
+    ${PROJECT_SOURCE_DIR}/third_party/mlir-emitc/include
+    ${PROJECT_BINARY_DIR}/third_party/mlir-emitc/include
+  )
+  add_definitions(-DIREE_HAVE_EMITC_DIALECT)
+endif()