SILGen: Forward declare SIL functions referenced by #_hasSymbol query helpers

[tshortli]

For a #_hasSymbol condition check, such as the following:

if #_hasSymbol(foo) { ... }

the compiler will generate a binary that implements something roughly equivalent to this pseudo-code:

// Call to helper function (and branch on its result) emitted during SILGen
if hasSymbolHelperForFoo() { ... }

func hasSymbolHelperForFoo() -> Bool {
  // Return true if the address of each linkable symbol associated with
  // the declaration `foo()` is non-null. This body is emitted by IRGen.
}

This pull request establishes the preconditions necessary for IRGen to do its part, which is to emit the function bodies for each #_hasSymbol helper function required by the program. Each unique declaration that needs an associated helper function is recorded on SILModule during SILGen and any SIL function prototypes that will be referenced by the helper function implementations are also emitted.

[tshortli]

@swift-ci please test

[nkcsgexi]

Super cool! I can now see how the proper layering of TBDGen could help this implementation.

[tshortli]

@swift-ci please test Windows

[tshortli]

@swift-ci please smoke test and merge

[tshortli]

@swift-ci please smoke test macOS

[artemcm]

Very nice!