[mlir] Support lowering of dialect attributes attached to top-level modules

This patch supports the processing of dialect attributes attached to top-level
module-type operations during MLIR-to-LLVMIR lowering.

This approach modifies the `mlir::translateModuleToLLVMIR()` function to call
`ModuleTranslation::convertOperation()` on the top-level operation, after its
body has been lowered. This, in turn, will get the
`LLVMTranslationDialectInterface` object associated to that operation's dialect
before trying to use it for lowering prior to processing dialect attributes
attached to the operation.

Since there are no `LLVMTranslationDialectInterface`s for the builtin and GPU
dialects, which define their own module-type operations, this patch also adds
and registers them. The requirement for always calling
`mlir::registerBuiltinDialectTranslation()` before any translation of MLIR to
LLVM IR where builtin module operations are present is introduced. The purpose
of these new translation interfaces is to succeed when processing module-type
operations, allowing the lowering process to continue and to prevent the
introduction of failures related to not finding such interfaces.

Differential Revision: https://reviews.llvm.org/D145932

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