Lifetime inference: restructure to support multiple dependencies.

Restructure the inference logic to allow the same function declaration to
independently annotate or infer different lifetime targets.

For example:

   @_lifetime(borrow a)
   /* DEFAULT: @_lifetime(b: copy b)
   func f(a: A, b: inout B) -> NE

The fact that we prevented this was somewhat accidental and surprising.

This restructuring simplifies the code but also gives us much more control over
the inference logic.

Fixes: rdar://159288750 ("A function cannot have a ~Escapable 'inout' parameter
in addition to other ~Escapable parameters" is not actionable)

include/swift/AST/LifetimeDependence.h

@@ -218,7 +218,7 @@ public:
         addressableParamIndicesAndImmortal(addressableParamIndices, isImmortal),
         conditionallyAddressableParamIndices(conditionallyAddressableParamIndices),
         targetIndex(targetIndex) {
-    assert(this->isImmortal() || inheritLifetimeParamIndices ||
+    ASSERT(this->isImmortal() || inheritLifetimeParamIndices ||
            scopeLifetimeParamIndices);
     ASSERT(!inheritLifetimeParamIndices ||
            !inheritLifetimeParamIndices->isEmpty());