[MLIR] Force generation of tabelgen'd headers before Dialects.

[stephenneuendorffer]

Summary

Dialects often depend on the public interface exposed from other
dialects, by including their header files. However, these header files
are often automatically generated. In order to avoid explicitly
specifying dialect dependencies in all dialect cmakefiles, we
(conservatively) assert that all automatically generated headers
need to be built before any dialect. This avoids build errors which
are difficult to identify because of incorrectly specified dependencies
and avoids transitive dependencies between dialects to be specified
in the build system.

[stephenneuendorffer]

This turns out not to be necessary

mlir/cmake/modules/AddMLIR.cmake

function(add_mlir_dialect dialect dialect_doc_filename)
   add_custom_target(${dialect_doc_filename}DocGen DEPENDS ${GEN_DOC_FILE})
   add_dependencies(mlir-doc ${dialect_doc_filename}DocGen)
 endfunction()
 
 # Declare the library associated with a dialect.
 function(add_mlir_dialect_library name)
   set_property(GLOBAL APPEND PROPERTY MLIR_DIALECT_LIBS ${name})
   add_llvm_library(${ARGV})
+  add_dependencies(${name} mlir-headers)
 endfunction(add_mlir_dialect_library)
 
 # Declare the library associated with a conversion.
 function(add_mlir_conversion_library name)
   set_property(GLOBAL APPEND PROPERTY MLIR_CONVERSION_LIBS ${name})
   add_llvm_library(${ARGV})
+  add_dependencies(${name} mlir-headers)
 endfunction(add_mlir_conversion_library)