SIL: Change the linker to deserialize more lazily

Note that SILModule::lookUpWitnessTable() only attempts to deserialize
the witness table if we already have a declaration. In the
SILLinkerVisitor, we would call lookUpWitnessTable(), see if it returned
null, create a declaration, and if deserializeLazily is true, we would
call lookUpWitnessTable() again; this time, there was a declaration, and
we would deserialize the witness table.

However, if there was already a witness table declaration when we first
called lookUpWitnessTable(), we would trigger deserialization, even if
deserializeLazily is false.

Change the first call to pass false.

I had to update a couple of tests; I believe they were already somewhat
nonsensical.

Author: slavapestov

lib/SIL/Linker.cpp

@@ -212,7 +212,7 @@ void SILLinkerVisitor::visitProtocolConformance(
   if (!VisitedConformances.insert(C).second)
     return;
 
-  SILWitnessTable *WT = Mod.lookUpWitnessTable(C, true);
+  SILWitnessTable *WT = Mod.lookUpWitnessTable(C, /*deserializeLazily=*/false);
 
   // If we don't find any witness table for the conformance, bail and return
   // false.
@@ -222,7 +222,7 @@ void SILLinkerVisitor::visitProtocolConformance(
     // Adding the declaration may allow us to now deserialize the body.
     // Force the body if we must deserialize this witness table.
     if (mustDeserialize) {
-      WT = Mod.lookUpWitnessTable(C, true);
+      WT = Mod.lookUpWitnessTable(C, /*deserializeLazily=*/true);
       assert(WT && WT->isDefinition()
              && "unable to deserialize witness table when we must?!");
     } else {

test/SIL/Serialization/Inputs/init_existential_inst_deserializes_witness_tables_input.swift

@@ -13,6 +13,7 @@ public struct X : P {
   public init() {}
 }
 
+@inlinable
 public func whatShouldIDo(_ p : P) {
   p.doSomething()
 }

test/SILOptimizer/dead_witness_module.swift

@@ -5,9 +5,11 @@
 // DeadFunctionElimination may not remove a method from a witness table which
 // is imported from another module.
 
+// FIXME: Ever since @usableFromInline began to be enforced, this test did not
+// test anything useful, and now the witness table is never deserialized at all.
+
 import TestModule
 
 testit(MyStruct())
 
-// CHECK: sil_witness_table public_external MyStruct: Proto module TestModule
-// CHECK-NEXT: method #Proto.confx!1: {{.*}} : @$s{{.*}}confx{{.*}}FTW
+// CHECK: sil_witness_table MyStruct: Proto module TestModule{{$}}