[TableGen] Fix infinite loop in SubstLeaves substitution

Previously we have `auto pos = std::string::find(...) != std::string::npos` as
if condition to control substring substitution. Instead of the position for the
found substring, `pos` will be a boolean value indicating found nor not. Then
used as the replace start position, we were always replacing starting from 0 or
1. If the replaced substring also has the pattern to be matched, we'll see
an infinite loop.

PiperOrigin-RevId: 235504681