Currently if an operation wants a C++ implemented parser/printer, it specifies inline
code blocks. This is quite problematic for various reasons, e.g. it requires defining
C++ inside of Tablegen which is discouraged when possible, but mainly because
nearly all usages simply forward to static functions (e.g. static void parseSomeOp(...))
with users devising their own standards for how these are defined.
This commit adds support for a hasCustomAssemblyFormat bit field that specifies if
a C++ parser/printer is needed, and when set to 1 declares the parse/print methods for
operations to override. For migration purposes, the existing behavior is untouched. Upstream
usages will be replaced in a followup to keep this patch focused on the new implementation.