[cxx-interop] Support SIMD types in reverse interop

This supports both built-in vector types and the SIMDX<T> Swift types
that are used through type aliases like float3. These SIMD types are
passed via compatibility structs (with their valued memcpy-d into the
ABI compatible struct) as they have special ABI rules. E.g., a float3 is
not passed as float3, but as float4 on the ABI level.

Previously, we did not expose simd types from Swift to C++ (unless the
SIMD types were impoted from clang).

rdar://153218744

test/Interop/SwiftToCxx/stdlib/swift-simd-in-cxx-execution.cpp

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend %S/swift-simd-in-cxx.swift -module-name SIMD -clang-header-expose-decls=all-public -typecheck -verify -emit-clang-header-path %t/simd.h
+
+// RUN: %target-interop-build-clangxx -c %s -I %t -o %t/swift-simd-execution.o
+// RUN: %target-interop-build-swift %S/swift-simd-in-cxx.swift -o %t/swift-simd-execution -Xlinker %t/swift-simd-execution.o -module-name SIMD -Xfrontend -entry-point-function-name -Xfrontend swiftMain
+
+// RUN: %target-codesign %t/swift-simd-execution
+// RUN: %target-run %t/swift-simd-execution | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: rdar157848231
+
+#include "simd.h"
+#include <assert.h>
+#include <iostream>
+
+int main() {
+  swift_float3 vec{1.0, 2.0, 3.0};
+  SIMD::swiftThingSIMD(vec);
+  // CHECK: SIMD3<Float>(1.0, 2.0, 3.0)
+  SIMD::swiftThingSIMD2(vec);
+  // CHECK: SIMD3<Float>(1.0, 2.0, 3.0)
+  swift_float3 vec2 = SIMD::swiftThingSIMD3();
+  std::cout << vec2.x << " " << vec2.y << " " << vec2.z << "\n";
+  // CHECK: 4 5 6
+  vec2 = SIMD::swiftThingSIMD4();
+  std::cout << vec2.x << " " << vec2.y << " " << vec2.z << "\n";
+  // CHECK: 4 5 6
+  return 0;
+}
+