[ADT][NFC] Simplify llvm::unique_function (#208251)
Refactor the core of UniqueFunctionBase to move away from static
function pointer tables, which consume ~4kiB in .data.rel.ro. Instead,
store two function pointers on construction of the UniqueFunctionBase,
one for call and one that combines move and destruction. Retain the
memcpy path for trivial inline structs. (I haven't verified whether this
is actually beneficial.)
Also, greatly simplify the class and implement the callbacks using
lambdas.
