Given code like the following:
```
auto kind = ConversionTarget::Kind{0};
CARBON_CHECK(!loc_id.is_valid(), "hello {0} world", kind);
```
Currently we would print 'hello <the next line>', as the check string
would be treated as terminating at the '{0}', so it does not print the
rest of the string or a newline. This is because ConversionTarget::Kind
is an enum with underlying type `int8_t` which is a char, and
llvm::formatv does not look if the type is an enum and treat is
specially. So it prints it as a char rather than a number, which in this
case is a nul terminator.
With this change, the '{0}' value will be converted to a larger integer
before being passed through to llvm::formatv so that char-sized enums
will print as a number, and the result is that we will print 'hello 0
world\n' as the developer intended.
Dana Jansens
c7ae2a7b18