add a new ImaginaryLiteral AST node that is used to

represent imaginary literals:

float _Complex A;
void foo() {
  A = 1.0iF;
}

generates:

  (BinaryOperator 0x2305ec0 '_Complex float' '='
    (DeclRefExpr 0x2305e60 '_Complex float' Decl='A' 0x2305cf0)
    (ImaginaryLiteral 0x2305f40 '_Complex float'
      (FloatingLiteral 0x2305ea0 'float' 1.000000))))

llvm-svn: 41413