[OpAsmParser] Refactor parseOptionalInteger to support wide integers, NFC.

OpAsmParser (and DialectAsmParser) supports a pair of
parseInteger/parseOptionalInteger methods, which allow parsing a bare
integer into a C type of your choice (e.g. int8_t) using templates.  It
was implemented in terms of a virtual method call that is hard coded to
int64_t because "that should be big enough".

Change the virtual method hook to return an APInt instead.  This allows
asmparsers for custom ops to parse large integers if they want to, without
changing any of the clients of the fixed size C API.

Differential Revision: https://reviews.llvm.org/D102120