IRGen: Emit capture descriptors in their own section

Now that reflection metadata is in TEXT, we can have relative references
to capture descriptors from closure context metadata.

Author: slavapestov

lib/IRGen/GenReflection.cpp

@@ -689,7 +689,8 @@ class CaptureDescriptorBuilder : public ReflectionMetadataBuilder {
                                         /*isConstant*/ true,
                                         llvm::GlobalValue::PrivateLinkage,
                                         init,
-                                        "capture_descriptor");
+                                        "\x01l__swift3_capture_descriptor");
+    var->setSection(IGM.getCaptureDescriptorMetadataSectionName());
     var->setAlignment(IGM.getPointerAlignment().getValue());
 
     auto replacer = llvm::ConstantExpr::getBitCast(var, IGM.Int8PtrTy);
@@ -731,6 +732,10 @@ std::string IRGenModule::getAssociatedTypeMetadataSectionName() {
   return getReflectionSectionName(*this, "assocty");
 }
 
+std::string IRGenModule::getCaptureDescriptorMetadataSectionName() {
+  return getReflectionSectionName(*this, "capture");
+}
+
 std::string IRGenModule::getReflectionStringsSectionName() {
   return getReflectionSectionName(*this, "reflstr");
 }

lib/IRGen/IRGenModule.h

@@ -632,6 +632,7 @@ class IRGenModule {
   std::string getBuiltinTypeMetadataSectionName();
   std::string getFieldTypeMetadataSectionName();
   std::string getAssociatedTypeMetadataSectionName();
+  std::string getCaptureDescriptorMetadataSectionName();
   std::string getReflectionStringsSectionName();
   std::string getReflectionTypeRefSectionName();