[Clang][Sema] Fix attribute((format)) bug on non-variadic functions

The [initial implementation][1] of __attribute__((format)) on non-variadic functions
accidentally only accepted one data argument. This worked:

```c
__attribute__((format(printf, 1, 2)))
void f(const char *, int);
```

but this didn't:

```c
__attribute__((format(printf, 1, 2)))
void f(const char *, int, int);
```

This is due to an oversight in changing the way diagnostics are emitted for
`attribute((format))`, and to a coincidence in the handling of the variadic case. Test
cases only covered the case that worked by coincidence.

Before the previous change, using `__attribute__((format))` on a non-variadic function at
all was an error and clang bailed out. After that change, it only generates a GCC
compatibility warning. However, as execution falls through, it hits a second diagnostic
when the first data argument is neither 0 nor the last parameter of the function.

This change updates that check to allow any parameter after the format string to be the
first data argument when the function is non-variadic. When the function is variadic, it
still needs to be the index of the `...` "parameter". Attribute documentation is updated
to reflect the change and new tests are added to verify that it works with _two_ data
parameters.

[1]: https://reviews.llvm.org/D112579

Radar-Id: rdar://102069446
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D137603