[LangRef] add note to warn-frame-size about ODR

[nickdesaulniers]

Summary

As sugguested by @dblaikie in D104342.

[dblaikie]

llvm/docs/LangRef.rst

2055–2057

That's probably not correct/feasible - ODR violations are "invalid, NDR" ie: pretty close to UB. Actually it'd be very easy for us to detect this (& we did in the old module metadata version) and reject it - so I think we've demonstrated that this is acceptable, but that we make no guarantees about which threshold will be respected.

C++-ish language would be to say "it's unspecified which threshold will be used when duplicate definitions are linked together"

[nickdesaulniers]

• update comment

[dblaikie]

Sounds alright

llvm/docs/LangRef.rst

``mustprogress``
     terminating loops without side-effects can be removed. If a `mustprogress`
     function does not satisfy this contract, the behavior is undefined.  This
     attribute does not apply transitively to callees, but does apply to call
     sites within the function. Note that `willreturn` implies `mustprogress`.
 ``"warn-stack-size"="<threshold>"``
     This attribute sets a threshold to emit diagnostics once the frame size is
     known should the frame size exceed the specified value.  It takes one
     required integer value, which should be a non-negative integer, and less
-    than `UINT_MAX`.
+    than `UINT_MAX`.  It's unspecified which threshold will be used when
+    duplicate definitions are linked together with differing values.
 ``vscale_range(<min>[, <max>])``
     This attribute indicates the minimum and maximum vscale value for the given
     function. A value of 0 means unbounded. If the optional max value is omitted
     then max is set to the value of min. If the attribute is not present, no
     assumptions are made about the range of vscale.
 
 Call Site Attributes
 ----------------------