SILGen: Ignore placeholders and missing methods during conformance emission.

Builds on #72286.

Use a more forgiving strategy when handling potentially invalid conformances
during SILGen. Conformances may be erroneously marked invalid during witness
resolution, even when they can still be successfully emitted, so we can't bail
out of witness table emission if the invalid bit is set. Instead, just ignore
placeholders and missing methods in the witness table emitter and trust that if
an error was diagnosed during resolution that compilation will be aborted.

Resolves rdar://125947349

Author: tshortli

lib/SILGen/SILGenType.cpp

@@ -554,11 +554,7 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     PrettyStackTraceConformance trace("generating SIL witness table",
                                       Conformance);
 
-    // Check whether the conformance is valid first.
     Conformance->resolveValueWitnesses();
-    if (Conformance->isInvalid())
-      return nullptr;
-
     auto *proto = Conformance->getProtocol();
     visitProtocolDecl(proto);
 
@@ -619,13 +615,12 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     return Conformance->getWitness(decl);
   }
 
-  void addPlaceholder(MissingMemberDecl *placeholder) {
-    llvm_unreachable("generating a witness table with placeholders in it");
-  }
-
-  void addMissingMethod(SILDeclRef requirement) {
-    llvm_unreachable("generating a witness table with placeholders in it");
-  }
+  // Treat placeholders and missing methods as no-ops. These may be encountered
+  // during lazy typechecking when SILGen triggers witness resolution and
+  // discovers and invalid conformance. The diagnostics emitted during witness
+  // resolution should cause compilation to fail.
+  void addPlaceholder(MissingMemberDecl *placeholder) {}
+  void addMissingMethod(SILDeclRef requirement) {}
 
   void addMethodImplementation(SILDeclRef requirementRef,
                                SILDeclRef witnessRef,