Implicit conversions for compound assignments.

[rsaill]

If I dump Clang's AST for the following program
(using clang -Xclang -ast-dump -fsyntax-only compound.c)

void foo(void){
  short s1 = 42;
  short s2 = 2;
  s1 <<= s2;
  return s1;
}

I can see the following information:

|-CompoundAssignOperator 0x125a308 <line:4:3, col:10> 'short' '<<=' ComputeLHSTy='int' ComputeResultTy='int'
    | |-DeclRefExpr 0x125a268 <col:3> 'short' lvalue Var 0x125a0e0 's1' 'short'
    | `-ImplicitCastExpr 0x125a2f0 <col:10> 'int' <IntegralCast>
    |   `-ImplicitCastExpr 0x125a2d8 <col:10> 'short' <LValueToRValue>
    |     `-DeclRefExpr 0x125a288 <col:10> 'short' lvalue Var 0x125a1b0 's2' 'short'

The ComputeLHSTy and ComputeResultTy parts tell me that when performing s1 <<= s2, s1 is implicitly converted to int before the shift
and the result is an int, so it should be re-converted to short before updating s1.

It would be nice to be able to extract this information from clangml'AST as well (at least when option 'Clang.Ast.Options.ignore_implicit_cast' is false).