[cxx-interop] Do not emit IR for C++20 requires expr

This fixes a compiler crash when emitting IR for a for-in loop over a C++ `std::vector` in C++20 mode.

rdar://108810356

Author: egorzhdan

lib/IRGen/GenClangDecl.cpp

@@ -119,6 +119,8 @@ class ClangDeclFinder
     return true;
   }
 
+  bool TraverseRequiresExpr(clang::RequiresExpr *RE) { return true; }
+
   // Do not traverse type locs, as they might contain expressions that reference
   // code that should not be instantiated and/or emitted.
   bool TraverseTypeLoc(clang::TypeLoc TL) { return true; }

test/Interop/Cxx/concepts/Inputs/method-requires.h

@@ -0,0 +1,11 @@
+inline void calledFromConceptBody(int x) {}
+inline void calledFromMethodBody(int x) {}
+
+struct MyStruct {
+  template <typename T>
+  void foo(T x)
+    requires requires(const T x) { calledFromConceptBody(x); }
+  {
+    calledFromMethodBody(x);
+  }
+};

test/Interop/Cxx/concepts/Inputs/module.modulemap

@@ -0,0 +1,4 @@
+module MethodRequires {
+  header "method-requires.h"
+  export *
+}

test/Interop/Cxx/concepts/method-requires.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swiftxx-frontend -emit-ir -Xcc -std=c++20 -I %S/Inputs %s | %FileCheck %s
+
+import MethodRequires
+
+var s = MyStruct()
+s.foo(123)
+// CHECK-NOT: calledFromConceptBody
+// CHECK: calledFromMethodBody

test/Interop/Cxx/stdlib/use-std-vector.swift

@@ -1,5 +1,5 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
-// FIXME: also test this in C++20 mode once rdar://108810356 is fixed.
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -std=c++20)
 //
 // REQUIRES: executable_test
 //