unnecessary Es after <expression> and mangling collisions between <expression> and <number>

[zygoloid]

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

• <exception-spec> = DO <expression> E
• <decltype> ::= Dt <expression> E
• <decltype> ::= DT <expression> E
• <template-arg> ::= X <expression> E

And something similar happens here:

• <array-type> ::= A <expression> _ <type>
• <builtin-type> ::= DB <expression> _
• <builtin-type> ::= DU <expression> _

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

[zygoloid]

(This should be tagged as ABI v2. These are too common to change now, except for DB / DU.)