[Runtime] Fix up incorrect attribute ordering

Clang no longer allows attributes before `extern "C"`. See
https://reviews.llvm.org/D126061.

There was *another* issue where GNU and standard attributes couldn't be
mixed, but that's been fixed by https://reviews.llvm.org/D137979.

Move the `SWIFT_RUNTIME_EXPORT` to the beginning so that `extern "C"` is
the first attribute.

Also remove an unnecessary include, which would prevent the compiler
itself being built on the released clang-15 (since the GNU/standard
attribute mixing fix isn't in clang-15).

Resolves #61468.

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_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
+SWIFT_RUNTIME_EXPORT SWIFT_RETURNS_NONNULL SWIFT_NODISCARD
 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_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
+SWIFT_RUNTIME_EXPORT SWIFT_RETURNS_NONNULL SWIFT_NODISCARD
 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_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
+SWIFT_RUNTIME_EXPORT SWIFT_RETURNS_NONNULL SWIFT_NODISCARD
 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_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
+SWIFT_RUNTIME_EXPORT SWIFT_RETURNS_NONNULL SWIFT_NODISCARD
 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_RETURNS_NONNULL SWIFT_NODISCARD SWIFT_RUNTIME_EXPORT
+SWIFT_RUNTIME_EXPORT SWIFT_RETURNS_NONNULL SWIFT_NODISCARD
 ValueMetadata *
 swift_allocateGenericValueMetadata(const ValueTypeDescriptor *description,
                                    const void *arguments,

lib/IRGen/GenDecl.cpp

@@ -30,7 +30,6 @@
 #include "swift/ClangImporter/ClangModule.h"
 #include "swift/Demangling/ManglingMacros.h"
 #include "swift/IRGen/Linking.h"
-#include "swift/Runtime/HeapObject.h"
 #include "swift/SIL/FormalLinkage.h"
 #include "swift/SIL/SILDebugScope.h"
 #include "swift/SIL/SILModule.h"