Big-endian fixes: various problems in ilasm

[uweigand]

While debugging problems running the roslyn test suite, I noticed a few more big-endian problems in ilasm:

• Byte-swap property and parameter default values of string type
  Default values of string type are swapped to little-endian by the parser (fieldInit). This has to be done there since "bytearray" values (which are provided by the user in little-endian format) are also encoded as string type, so we won't be able to tell the two cases apart later. However, the MD layer expects default values of all types, including strings, to be in native byte order (they're swapped again on big-endian hosts in CMiniMdBase::SwapConstant). In order to fix this mismatch, the default values of fields are already swapped yet another time by ilasm code in Assembler::EmitField. However, that same problem exists for default values of parameters or properties. This patch adds an equivalent swap operation there.

• Prepare custom attribute blobs in little-endian byte order
  Custom attribute blobs (as opposed to default values) are expected to be provided in little-endian order by the MD layer. This already works for attributes specifies as byte arrays. But for attributes specified as structured values, values of all types except strings are currently provided in native order by the parser -- this is because serInit shares most of its implementation with fieldInit above. To fix this, I've added a new helper AppendFieldToCustomBlob that copies a typed value over to the custom attribute blob, byte-swapping as necessary. There are also a couple of places where ilasm synthesizes attribute blobs; some of those had a few byte swaps missing as well (see AsmMan::EmitDebuggableAttribute and the PermissionDecl constructor).

• Fix byte order of ELEMENT_TYPE_TYPEDEF typespec blobs and of VTable blobs
  These two are unrelated to the above, just another set of instances where byte swaps were missing.

[uweigand]

Hi @BruceForstall , can you have a look?

[BruceForstall]

/azp run runtime-coreclr ilasm

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[BruceForstall]

LGTM