nothrow and const Attribtues

[Aaron Ballman]

The nothrow and const attributes have a curious construct that does
not appear with any other attributes. Both of them check whether the
attribute has already been applied, and if it has, it silently fails
to reapply the attribute.  Eg)

  if (NoThrowAttr *Existing = D->getAttr<NoThrowAttr>()) {
    if (Existing->getLocation().isInvalid())
      Existing->setRange(Attr.getRange());
  } else {
    D->addAttr(::new (S.Context)
               NoThrowAttr(Attr.getRange(), S.Context,
                           Attr.getAttributeSpellingListIndex()));
  }

While this makes some sense, two things in particular are worrisome:

1) We lose syntactic information because we are losing the as-written
attribute information. This happens relatively frequently, but any
time we can cut that down would be good.
2) This is not consistently applied across attributes.

I am wondering whether it is reasonable to remove the check for the
existing attribute from the nothrow and const attributes, and simply
apply multiple times. Since neither of these attributes accept
arguments, it seems like this would be a harmless action. However, I
may not have the full picture; this is the original commit message:

dgregor  6/15/2011 1:45:11 AM
Don't add redundant FormatAttr, ConstAttr, or NoThrowAttr attributes,
either imlicitly (for builtins) or explicitly (due to multiple
specification of the same attributes). Fixes <rdar://problem/9612060>.

(Note, I am not suggesting any modifications for FormatAttr.)

Thanks!

~Aaron