David Blaikie bb9942823f DebugInfo: Emit as "C++" rather than "C" (#6361) ...

This helps at least lldb handle calling functions (currently the debug
info describes every function as `void()`, so no parameters or return
values are supported) - seems gdb and lldb both depend on demangling to
varying degrees in C code (marking a function as "prototyped" in C in
DWARF does seem to also address this problem).

Given:
```
fn PrintThree() {
  Core.Print(3);
}
```
Before:
```
  (lldb) p PrintThree()
  error: Couldn't look up symbols:
    PrintThree
  Hint: The expression tried to call a function that is not present in
    the target, perhaps because it was optimized out by the compiler.
```
After:
```
  (lldb) p PrintThree()
  3
  (lldb)
```

2025-11-18 18:28:56 +00:00

..

simple.carbon

DebugInfo: Emit as "C++" rather than "C" (#6361)

2025-11-18 18:28:56 +00:00