Add a shorthand method to ThreadState to get a pointer to the active
FunctionState if there is one, or a nullptr otherwise.
When determining alloca visibility in seec::trace::FunctionState, if the
llvm.dbg.declare associated with an alloca is the very next Instruction, then
treat the alloca as visible.
Add methods to query if an Instruction has Stmt mapping information, and if its
Stmt mapping information is equal to another Instruction's, without needing to
lookup the Stmt itself (simply compare the metadata pointers as the metadata is
equal and thus uniqued by LLVM).
Add functions to query the next Instruction that will become active in the
currently active Function, or the previous Instruction that was active, for a
given ThreadState.
Modify forward movement of SeeC-Clang mapped thread states to attempt to move
in logical steps, rather than exposing the underlying LLVM IR state changes to
the user.