Change the contract with the type/attribute parsing to let the dispatch handle the mnemonic

This breaking change requires to remove printing the mnemonic in the print()
method on Type/Attribute classes.
This makes it consistent with the parsing code which alread handles the
mnemonic outside of the parsing method.

This likely won't break the build for anyone, but tests will start
failing for dialects downstream. The fix is trivial and look like
going from:

void emitc::OpaqueType::print(DialectAsmPrinter &printer) const {
  printer << "opaque<\"";

to:

void emitc::OpaqueAttr::print(DialectAsmPrinter &printer) const {
  printer << "<\"";

Reviewed By: rriddle, aartbik

Differential Revision: https://reviews.llvm.org/D113334