[cxx-interop] Prevent protocol conformance table from being prematurely populated

We've started seeing build failures where the conformance of a `std::vector` instantiation to `CxxVector` is missing. This was because the LifetimeDependenceInfoRequest triggers the protocol conformance table for the instantiation to be built before the synthesized conformance gets added.

This works around the issue by preventing LifetimeDependenceInfoRequest from running for the synthesized default argument generator function, which was the culprit of this particular failure.

rdar://155977071

Author: egorzhdan

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -21,6 +21,7 @@
 #include "swift/AST/ParameterList.h"
 #include "swift/AST/Pattern.h"
 #include "swift/AST/Stmt.h"
+#include "swift/AST/TypeCheckRequests.h"
 #include "swift/Basic/Assertions.h"
 #include "clang/AST/Mangle.h"
 #include "clang/Sema/DelayedDiagnostic.h"
@@ -2532,6 +2533,11 @@ SwiftDeclSynthesizer::makeDefaultArgument(const clang::ParmVarDecl *param,
   funcDecl->setAccess(AccessLevel::Public);
   funcDecl->getAttrs().add(new (ctx)
                                AlwaysEmitIntoClientAttr(/*IsImplicit=*/true));
+  // At this point, the parameter/return types of funcDecl might not be imported
+  // into Swift completely, meaning that their protocol conformances might not
+  // be populated yet. Prevent LifetimeDependenceInfoRequest from prematurely
+  // populating the conformance table for the types involved.
+  ctx.evaluator.cacheOutput(LifetimeDependenceInfoRequest{funcDecl}, {});
 
   ImporterImpl.defaultArgGenerators[param] = funcDecl;