unnecessary `E`s after <expression> and mangling collisions between <expression> and <number>

We require \ to be self-delimiting, but also sometimes follow an \ with an unnecessary E. We should remove those. This happens here:

\ = DO \ E
\ ::= Dt \ E
\ ::= DT \ E
\ ::= X \ E

And something similar happens here:

\ ::= A \ _ \
\ ::= DB \ _
\ ::= DU \ _

These last three manglings are also broken for another reason: they collide with the corresponding manglings with the \ replaced by a \, where the mangling could also be for an \. We could fix them by moving the _ to before the \.

itanium-cxx-abi / cxx-abi

unnecessary `E`s after <expression> and mangling collisions between <expression> and <number> #164