Conversion to LLVM Dialect: integrate TypeConverter into LLVMLowering

Conversion to LLVM Dialect: integrate TypeConverter into LLVMLowering

    Historically, the conversion from standard and built-in types to the LLVM IR
    dialect types was performed by a dedicated class, TypeConverter.  This class
    served to contain references to the LLVM IR dialect and to the LLVM IR Module
    to allow querying the data layout.  Recently, the LLVMLowering class was
    introduced to make the conversion to the LLVM IR dialect extensible to other
    source dialects.  This class also includes the references to the LLVM IR
    dialect and module.  TypeConverter was extended with basic support for
    dialect-specific type conversion through callbacks.  This is not sufficient in
    cases where dialect-specific types appear inside other types, such as function
    or container types.

    Integrate TypeConverter into LLVMLowering.  Whenever a subtype needs to be
    converted during standard type conversion (e.g. an argument or a result of a
    FunctionType), the conversion will call to the virtual function
    `LLVMLowering::convertType`, which can be extended to support dialect-specific
    types.

    Provide a new LLVMOpConversion class that serves as a base class for all
    conversions to the LLVM IR dialect and gives them access to LLVMLowering for
    the purpose of type conversion.  Update Linalg to LLVM IR lowering to use this
    class.

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PiperOrigin-RevId: 247407314