[stdlib] Fix cc mismatch violation on swift_isClassType

_isClassType is mainly called by compiler-generated code, but it's also
called by stdlib. The call-site of stdlib used @_silgen_name to link the
function, so it's called through swiftcc. However its cc is defined as
C-cc, so caller and callee used different cc.
This patch adds C-compatible decl of swift_isClassType in shims so that
it can be called by stdlib with C-cc.
This change doesn't break ABI because `_isClassType` in stdlib was
defined as internal API.

Author: kateinoigakukun

stdlib/public/SwiftShims/RuntimeShims.h

@@ -20,6 +20,7 @@
 
 #include "SwiftStddef.h"
 #include "SwiftStdint.h"
+#include "SwiftStdbool.h"
 #include "Visibility.h"
 
 #ifdef __cplusplus
@@ -61,6 +62,12 @@ int _swift_stdlib_putc_stderr(int C);
 SWIFT_RUNTIME_STDLIB_API
 __swift_size_t _swift_stdlib_getHardwareConcurrency(void);
 
+#ifdef __swift__
+/// Called by ReflectionMirror in stdlib through C-calling-convention
+SWIFT_RUNTIME_STDLIB_API
+__swift_bool swift_isClassType(const void *type);
+#endif
+
 /// Manually allocated memory is at least 16-byte aligned in Swift.
 ///
 /// When swift_slowAlloc is called with "default" alignment (alignMask ==

stdlib/public/core/ReflectionMirror.swift

@@ -12,8 +12,11 @@
 
 import SwiftShims
 
-@_silgen_name("swift_isClassType")
-internal func _isClassType(_: Any.Type) -> Bool
+internal func _isClassType(_ type: Any.Type) -> Bool {
+  // a thick metatype is represented with a pointer metadata structure,
+  // so this unsafeBitCast is a safe operation here.
+  return swift_isClassType(unsafeBitCast(type, to: UnsafeRawPointer.self))
+}
 
 @_silgen_name("swift_getMetadataKind")
 internal func _metadataKind(_: Any.Type) -> UInt