Dialect conversion: decouple function signature conversion from type conversion

Function types are built-in in MLIR and affect the validity of the IR itself.
However, advanced target dialects such as the LLVM IR dialect may include
custom function types.  Until now, dialect conversion was expecting function
types not to be converted to the custom type: although the signatures was
allowed to change, the outer type must have been an mlir::FunctionType.  This
effectively prevented dialect conversion from creating instructions that
operate on values of the custom function type.

Dissociate function signature conversion from general type conversion.
Function signature conversion must still produce an mlir::FunctionType and is
used in places where built-in types are required to make IR valid.  General
type conversion is used for SSA values, including function and block arguments
and function results.

Exercise this behavior in the LLVM IR dialect conversion by converting function
types to LLVM IR function pointer types.  The pointer to a function is chosen
to provide consistent lowering of higher-order functions: while it is possible
to have a value of function type, it is not possible to create a function type
accepting a returning another function type.

PiperOrigin-RevId: 234124494