Simplify the way spirv::StructTypes are parsed.
The existing logic to parse spirv::StructTypes is very brittle. This
change simplifies the parsing logic a lot. The simplification also
allows for memberdecorations to be separated by commas instead of
spaces (which was an artifact of the existing parsing logic). The
change also needs a modification to mlir::parseType to return the
number of chars parsed. Adding a new parseType method to do so.
Also allow specification of spirv::StructType with no members.