Revert "fix wrong attribute ordering in the runtime"

This reverts commit 0494db0 as it was a
workaround for a Clang bug that is now fixed (see
https://reviews.llvm.org/D137979).

Author: bnbarham

include/swift/Runtime/Heap.h

@@ -29,7 +29,7 @@ namespace swift {
 // Never returns nil. The returned memory is uninitialized. 
 //
 // An "alignment mask" is just the alignment (a power of 2) minus 1.
-SWIFT_EXTERN_C SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT_ATTRIBUTE
+SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
 void *swift_slowAlloc(size_t bytes, size_t alignMask);
 
 // If the caller cannot promise to zero the object during destruction,

include/swift/Runtime/HeapObject.h

@@ -61,7 +61,7 @@ struct OpaqueValue;
 ///
 /// POSSIBILITIES: The argument order is fair game.  It may be useful
 /// to have a variant which guarantees zero-initialized memory.
-SWIFT_EXTERN_C SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT_ATTRIBUTE
+SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
 HeapObject *swift_allocObject(HeapMetadata const *metadata,
                               size_t requiredSize,
                               size_t requiredAlignmentMask);
@@ -117,7 +117,7 @@ BoxPair swift_makeBoxUnique(OpaqueValue *buffer, Metadata const *type,
                                     size_t alignMask);
 
 /// Returns the address of a heap object representing all empty box types.
-SWIFT_EXTERN_C SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT_ATTRIBUTE
+SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
 HeapObject* swift_allocEmptyBox();
 
 /// Atomically increments the retain count of an object.

include/swift/Runtime/Metadata.h

@@ -313,7 +313,7 @@ swift_getGenericMetadata(MetadataRequest request,
 ///   - installing new v-table entries and overrides; and
 ///   - registering the class with the runtime under ObjC interop.
 /// Most of this work can be achieved by calling swift_initClassMetadata.
-SWIFT_EXTERN_C SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT_ATTRIBUTE
+SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
 ClassMetadata *
 swift_allocateGenericClassMetadata(const ClassDescriptor *description,
                                    const void *arguments,
@@ -322,7 +322,7 @@ swift_allocateGenericClassMetadata(const ClassDescriptor *description,
 /// Allocate a generic value metadata object.  This is intended to be
 /// called by the metadata instantiation function of a generic struct or
 /// enum.
-SWIFT_EXTERN_C SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT_ATTRIBUTE
+SWIFT_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
 ValueMetadata *
 swift_allocateGenericValueMetadata(const ValueTypeDescriptor *description,
                                    const void *arguments,

stdlib/public/SwiftShims/swift/shims/Visibility.h

@@ -217,13 +217,10 @@
 // TODO: use this in shims headers in overlays.
 #if defined(__cplusplus)
 #define SWIFT_EXPORT_FROM(LIBRARY) extern "C" SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
-#define SWIFT_EXTERN_C extern "C" 
 #else
 #define SWIFT_EXPORT_FROM(LIBRARY) SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
-#define SWIFT_EXTERN_C
 #endif
 #define SWIFT_RUNTIME_EXPORT SWIFT_EXPORT_FROM(swiftCore)
-#define SWIFT_RUNTIME_EXPORT_ATTRIBUTE SWIFT_EXPORT_FROM_ATTRIBUTE(swiftCore)
 
 #if __cplusplus > 201402l && __has_cpp_attribute(fallthrough)
 #define SWIFT_FALLTHROUGH [[fallthrough]]