Add Embedded Swift Cxx exception personality

Users frequently run into a missing runtime symbol for Cxx exceptions
(`_swift_exceptionPersonality`) when mixing Embedded Swift and Cxx with
exceptions enabled. This leads to a confusing an hard to debug linker
error. This commit adds an implementation of this function to the
Embedded Swift runtime which simply fatal errors if a cxx exception is
caught in a Swift frame.

Issue: rdar://164423867
Issue: #85490

stdlib/public/core/EmbeddedRuntime.swift

@@ -432,7 +432,7 @@ func isValidPointerForNativeRetain(object: Builtin.RawPointer) -> Bool {
   #if _pointerBitWidth(_64)
   if unsafe (objectBits & HeapObject.immortalObjectPointerBit) != 0 { return false }
   #endif
-  
+
   return true
 }
 
@@ -750,3 +750,19 @@ func _embeddedReportFatalErrorInFile(prefix: StaticString, message: UnsafeBuffer
   if message.count > 0 { print(": ", terminator: "") }
   unsafe print(message)
 }
+
+// CXX Exception Personality
+
+public typealias _Unwind_Action = CInt
+public typealias _Unwind_Reason_Code = CInt
+
+@c @used
+public func _swift_exceptionPersonality(
+  version: CInt,
+  actions: _Unwind_Action,
+  exceptionClass: UInt64,
+  exceptionObject: UnsafeMutableRawPointer,
+  context: UnsafeMutableRawPointer
+) -> _Unwind_Reason_Code {
+  fatalError("C++ exception handling detected but the Embedded Swift runtime does not support exceptions")
+}

test/embedded/linkage/cxx-exceptions.swift

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -enable-experimental-feature Embedded -O -c -o %t/main.o -cxx-interoperability-mode=default
+// RUN: %target-clang %target-clang-resource-dir-opt %t/main.o -o %t/a.out -dead_strip
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: executable_test
+// REQUIRES: swift_feature_Embedded
+
+@_expose(Cxx)
+func f1() {
+  print("OK!")
+}
+
+f1()
+
+// CHECK: OK!