[FuncSpec][NFC] Refactor internal structures.

`ArgInfo` is reduced to only contain a pair of {formal,actual} values.
The specialized function `Fn` and the `Partial` flag are redundant in
this structure. The `Gain` is moved to a new struct `SpecializationInfo`.

The value mappings created by cloneCandidateFunction() are being used
by rewriteCallSites() for matching the formal arguments of recursive
functions.

The list of specializations is passed by reference to calculateGains()
instead of being returned by value.

The `IsPartial` flag is removed from isArgumentInteresting() and
getPossibleConstants() as it's no longer used anywhere in the code.

Differential Revision: https://reviews.llvm.org/D120753