out parameter diagnostics affect uninitialized variables at the _call site_

[MarijnS95]

A recent PR to prevent leaving out parameters uninitialized in function calls (#5047) seems to have escaped its cage and is now also disallowing uninitialized arguments into functions with out paramameters. See for example this (IMO totally valid) Interlocked* invocation:

        uint original;
        buffer.InterlockedAdd(offset, value, original);
        return original;

bindless.hlsl:110:46: error: variable 'original' is uninitialized when used here [-Werror,-Wuninitialized]
        buffer.InterlockedAdd(offset, value, original);
                                             ^~~~~~~~
bindless.hlsl:109:22: note: initialize the variable 'original' to silence this warning
        uint original;
                     ^
                      = 0

(Compiler from 667fb77)

I thought the whole point of out parameters and by extension this diagnostic is to know and force that the function will initialize original here, and it should thus not need to be pre-initialized by the caller.

Originally posted by @MarijnS95 in #5047 (comment), cc @llvm-beanz

[llvm-beanz]

Just going to drop a full-source example here:

RWByteAddressBuffer buffer;

void interlockWrapper(out uint original) {
  buffer.InterlockedAdd(16, 1, original);
}

[llvm-beanz]

Something to also be aware of here. -Wuninitialized in DXC does cause spurious warnings around struct types too. We also have issue #2494, which is caused by our bad AST hygiene.