A pragma for ImportC allows to set `nothrow`, `@nogc` or `pure`

The following new pragma for ImportC allows to set default storage
classes for function declarations:
```c
#pragma attribute(push, [storage classes...])
```
The storage classes `nothrow`, `nogc` and `pure` are supported.
Unrecognized attributes are ignored.
Enabling a default storage class affects all function declarations
after the pragma until it is disabled with another pragma.
Declarations in includes are also affected.
The changed storage classes are pushed on a stack. The last change can
be undone with the following pragma.
The following example
enables `@nogc` and `nothrow` for a library:

```c
#pragma attribute(push, nogc, nothrow)
#include <somelibrary.h>
#pragma attribute(pop)
```

This can also disable multiple default storage classes at the same time,
if they were enabled with a single `#pragma attribute(push, ...)` directive.