[PATCH] Symbol offsets

Quoting Peter:

Before, when I was thinking about calls to functions with prefix data in
cases where the function entry point appears after the data (i.e. GHC's use
case), I was imagining that we could have two new properties for functions:
the symbol offset and the entry point offset. UBSan etc would set both to
zero, while GHC would set the former to zero and the latter to the size of
the prefix.

Agree, but let me voice a heretic opinion: maybe those annotations
should not go on functions but on basic blocks! After all they make
little sense when a function has no entry block. Also in presence of
computed gotos this type of prologue and metadata on blocks would be
very interesting to have. I'm thinking of multiple-return points (akin
to exceptions, but lighter weight). I have no proposal to make, just
thinking out loud.