FreeBSD: Autolink C++ runtime

This hooks up the autolink mechanism to link the C++ runtime when C++
interop is enabled on FreeBSD.

Author: etcwilde

lib/AST/ModuleDependencies.cpp

@@ -573,7 +573,8 @@ void swift::dependencies::registerCxxInteropLibraries(
                       return mainModuleName == Name;
                     })) {
     // Only link with CxxStdlib on platforms where the overlay is available.
-    if (Target.isOSDarwin() || Target.isOSLinux() || Target.isOSWindows())
+    if (Target.isOSDarwin() || Target.isOSLinux() || Target.isOSWindows() ||
+        Target.isOSFreeBSD())
       RegistrationCallback(LinkLibrary{"swiftCxxStdlib", LibraryKind::Library,
                                        hasStaticCxxStdlib});
   }

lib/Frontend/CompilerInvocation.cpp

@@ -391,7 +391,8 @@ void CompilerInvocation::computeCXXStdlibOptions() {
     // (see https://reviews.llvm.org/D101479).
     LangOpts.CXXStdlib = CXXStdlibKind::Msvcprt;
     LangOpts.PlatformDefaultCXXStdlib = CXXStdlibKind::Msvcprt;
-  } else if (LangOpts.Target.isOSLinux() || LangOpts.Target.isOSDarwin()) {
+  } else if (LangOpts.Target.isOSLinux() || LangOpts.Target.isOSDarwin() ||
+             LangOpts.Target.isOSFreeBSD()) {
     auto [clangDriver, clangDiagEngine] =
         ClangImporter::createClangDriver(LangOpts, ClangImporterOpts);
     auto clangDriverArgs = ClangImporter::createClangArgs(