AsmStmt has no accessor for assembly, or clobberlist

Given an AsmStmt as follows:
```c
asm volatile
(
"syscall"
: "=a" (ret)
: "0"(nr), "D"(arg0), "S"(arg1), "d"(arg2)
: "rcx", "r11", "memory"
);
```
There seems to be no predicate to retrieve the assembly string (in this case, `"syscall"`), the return or the clobberlist. Calling `getAChild` inherited from `Expr` only yields the input/output expressions.