[cxx-interop] Allow compiling with libc++ on Linux

This makes sure that Swift respects `-Xcc -stdlib=libc++` flags.

Clang already has existing logic to discover the system-wide libc++ installation on Linux. We rely on that logic here.

Importing a Swift module that was built with a different C++ stdlib is not supported and emits an error.

The Cxx module can be imported when compiling with any C++ stdlib. The synthesized conformances, e.g. to CxxRandomAccessCollection also work. However, CxxStdlib currently cannot be imported when compiling with libc++, since on Linux it refers to symbols from libstdc++ which have different mangled names in libc++.

rdar://118357548 / https://github.com/swiftlang/swift/issues/69825

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 881; // Changes to LifetimeDependence
+const uint16_t SWIFTMODULE_VERSION_MINOR = 882; // CXXStdlibKind
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -949,6 +949,7 @@ namespace options_block {
     HAS_CXX_INTEROPERABILITY_ENABLED,
     ALLOW_NON_RESILIENT_ACCESS,
     SERIALIZE_PACKAGE_ENABLED,
+    CXX_STDLIB_KIND,
   };
 
   using SDKPathLayout = BCRecordLayout<
@@ -1032,6 +1033,11 @@ namespace options_block {
     HAS_CXX_INTEROPERABILITY_ENABLED
   >;
 
+  using CXXStdlibKindLayout = BCRecordLayout<
+    CXX_STDLIB_KIND,
+    BCFixed<2>
+  >;
+
   using AllowNonResilientAccess = BCRecordLayout<
     ALLOW_NON_RESILIENT_ACCESS
   >;