Sometimes, as an optimization, compilers will lift out known-constant
function parameters from the actual calling convention. This is represented
by function argument debuginfo (DW_TAG_formal_parameter) having a
DW_AT_const_value attribute instead of a DW_AT_location.
In order for dtrace to understand such functions, we need to represent it
somehow in CTF. This is one scheme for doing so. An alternative might be
setting some sort of flag on the FUNCTION object (or adding a FUNCTION2
type) and adding the reordering and constant elimination logic there.
This commit does not add dtrace support for this new CONSTVAL type, just
ctfconvert, ctfmerge, and ctfdump.