[flang] Fix missing generated includes in out of tree build

[DavidSpickett]

Summary

875fd9df76ded4a88a3a44b690f290ea98f91705 added a new dialect
with some generated files.

When flang is built out of tree (build llvm/clang/mlir first, then
build flang pointing at the first build) those files were not created
at all.

I don't 100% understand why not but juding by the comment at the top
of the file, add_mlir_interface probably expects to run in an MLIR
directory, as add_mlir_dialect does.

So in the same way, I've just inlined enough of that function to
fix the out of tree build.

[jeanPerier]

Thanks a lot for identifying and fixing the issue !

flang/include/flang/Optimizer/Dialect/CMakeLists.txt

 # This replicates part of the add_mlir_dialect cmake function from MLIR that
 # cannot be used her because it expects to be run inside MLIR directory which
 # is not the case for FIR.
 set(LLVM_TARGET_DEFINITIONS FIRAttr.td)
 mlir_tablegen(FIREnumAttr.h.inc -gen-enum-decls)
 mlir_tablegen(FIREnumAttr.cpp.inc -gen-enum-defs)
 mlir_tablegen(FIRAttr.h.inc --gen-attrdef-decls)
 mlir_tablegen(FIRAttr.cpp.inc -gen-attrdef-defs)
 
 set(LLVM_TARGET_DEFINITIONS FIROps.td)
 mlir_tablegen(FIROps.h.inc -gen-op-decls)
 mlir_tablegen(FIROps.cpp.inc -gen-op-defs)
 mlir_tablegen(FIROpsTypes.h.inc --gen-typedef-decls)
 mlir_tablegen(FIROpsTypes.cpp.inc --gen-typedef-defs)
-add_mlir_interface(FortranVariableInterface)
 add_public_tablegen_target(FIROpsIncGen)
 
+set(LLVM_TARGET_DEFINITIONS FortranVariableInterface.td)
+mlir_tablegen(FortranVariableInterface.h.inc -gen-op-interface-decls)
+mlir_tablegen(FortranVariableInterface.cpp.inc -gen-op-interface-defs)
+
 set(LLVM_TARGET_DEFINITIONS CanonicalizationPatterns.td)
 mlir_tablegen(CanonicalizationPatterns.inc -gen-rewriters)
 add_public_tablegen_target(CanonicalizationPatternsIncGen)
 
 add_custom_target(flang-doc)
 set(dialect_doc_filename "FIRLangRef")
 
 set(LLVM_TARGET_DEFINITIONS FIROps.td)