String -> NativeString conversion incorrectly omitted

[juliusikkala]

NativeStrings are incorrectly assumed to be equivalent to Strings, without going through the proper constructors. This causes compilation errors in e.g. the following program:

void string_conversion(String title)
{
    NativeString c_title = NativeString(title);
    printf(c_title);
}

export __extern_cpp int main(int argc, int8_t** argv)
{
    string_conversion("asdf");
    return 0;
}

Compiled with slangc -target exe test.slang -o test:

gcc 13.3: testi.slang(3): error :  cannot convert ‘Slang::String’ to ‘const char*’
    3 | void string_conversion(String title)
      |            ^~~~~~~
      |            |
      |            Slang::String
gcc 13.3: /usr/include/x86_64-linux-gnu/bits/stdio2.h(84): note :    initializing argument 1 of ‘int printf(const char*, ...)’
   84 | printf (const char *__restrict __fmt, ...)
      |         ~~~~~~~~~~~~~~~~~~~~~~~^~~~~

The generated code looks like this:

#line 1 "test.slang"
static void string_conversion_0(String title_0)
{

#line 1
    printf(title_0);



    return;
}


#line 7
int32_t main(int32_t argc_0, int8_t * * argv_0)
{
    string_conversion_0(Slang::toTerminatedSlice("asdf"));
    return int(0);
}

I have a PR lined up for this, but I'm not 100% sure if it's the correct solution, so I made this issue to keep track of this in case the PR isn't the right way to fix this.