Add a macro to opt into aggressive constprop (#38080)
Right now aggressive constprop is essentially tied to the inlining
threshold (or to their name being `getproperty` or `setproperty!`
respectively, which can be both somewhat brittle if the inlining cost
changes and insufficient when you do really know that const prop
would be beneficial even if the function is not inlineable. This
adds a simple macro that can be used to manually annotate methods
to force aggressive constprop on them.
