Make SILArgumentConvention a "method"-enum.

[gottesmm]

This means using a struct so we can put methods on the struct and using an
anonymous enum to create namespaced values. Specifically:

struct SILArgumentConvention {
  enum : uint8_t {
    Indirect_In,
    Indirect_In_Guaranteed,
    Indirect_Inout,
    Indirect_InoutAliasable,
    Indirect_Out,
    Direct_Owned,
    Direct_Unowned,
    Direct_Deallocating,
    Direct_Guaranteed,
  } Value;

  SILArgumentConvention(decltype(Value) NewValue)
      : Value(NewValue) {}

  operator decltype(Value)() const {
    return Value;
  }

  ParameterConvention getParameterConvention() const {
    switch (Value) {
    ...
    }
  }

  bool isIndirectConvention() const {
    ...
  }
};

This allows for:

1. Avoiding abstraction leakage via the enum type. If someone wants to use
   decltype as well, I think that is enough work that the leakage is acceptable.
2. Still refer to enum cases like we are working with an enum class
   (e.g. SILArgumentConvention::Direct_Owned).
3. Avoid using the anonymous type in function arguments due to an implicit
   conversion.
4. And most importantly... drum roll add methods to our enums!

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[gottesmm]

@swift-ci Please smoke test and merge