IRGen: Weak-link opaque type entry points.

When backward deploying to an OS that may not have these entry points, weak-link them so that they
can be used conditionally in availability contexts that check for them.

rdar://problem/50731151

include/swift/AST/ASTContext.h

@@ -50,6 +50,7 @@ namespace clang {
 namespace swift {
   class ASTContext;
   enum class Associativity : unsigned char;
+  class AvailabilityContext;
   class BoundGenericType;
   class ClangNode;
   class ConstructorDecl;
@@ -584,6 +585,9 @@ class ASTContext final {
   void addDestructorCleanup(T &object) {
     addCleanup([&object]{ object.~T(); });
   }
+
+  /// Get the runtime availability of the opaque types language feature for the target platform.
+  AvailabilityContext getOpaqueTypeAvailability();
 
   //===--------------------------------------------------------------------===//
   // Diagnostics Helper functions

include/swift/Runtime/RuntimeFnWrappersGen.h

@@ -20,6 +20,8 @@
 #include "llvm/ADT/ArrayRef.h"
 
 namespace swift {
+
+class AvailabilityContext;
 
 /// Generate an llvm declaration for a runtime entry with a
 /// given name, return types, argument types, attributes and
@@ -28,6 +30,7 @@ llvm::Constant *getRuntimeFn(llvm::Module &Module,
                       llvm::Constant *&cache,
                       char const *name,
                       llvm::CallingConv::ID cc,
+                      bool isWeakLinked,
                       llvm::ArrayRef<llvm::Type*> retTypes,
                       llvm::ArrayRef<llvm::Type*> argTypes,
                       llvm::ArrayRef<llvm::Attribute::AttrKind> attrs);