Cxx: build Cxx module statically on all platforms

This adjusts Cxx to be built statically on all platforms including
Windows.  The static library support is sufficient to support this
module linking statically on Windows.

Author: compnerd

lib/IRGen/GenDecl.cpp

@@ -488,8 +488,15 @@ void IRGenModule::emitSourceFile(SourceFile &SF) {
       this->addLinkLibrary(LinkLibrary("stdc++", LibraryKind::Library));
 
     // Do not try to link Cxx with itself.
-    if (!getSwiftModule()->getName().is("Cxx"))
-      this->addLinkLibrary(LinkLibrary("swiftCxx", LibraryKind::Library));
+    if (!getSwiftModule()->getName().is("Cxx")) {
+      bool isStatic = false;
+      if (const auto *M = Context.getModuleByName("Cxx"))
+        isStatic = M->isStaticLibrary();
+      this->addLinkLibrary(LinkLibrary(target.isOSWindows() && isStatic
+                                          ? "libswiftCxx"
+                                          : "swiftCxx",
+                                       LibraryKind::Library));
+    }
 
     // Do not try to link CxxStdlib with the C++ standard library, Cxx or
     // itself.

stdlib/public/Cxx/CMakeLists.txt

@@ -1,17 +1,12 @@
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/../../cmake/modules)
 include(StdlibOptions)
 
-set(SWIFT_CXX_LIBRARY_KIND STATIC)
-if("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WINDOWS")
-  set(SWIFT_CXX_LIBRARY_KIND SHARED)
-endif()
-
 set(SWIFT_CXX_DEPS symlink_clang_headers)
 if(SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT)
   list(APPEND SWIFT_CXX_DEPS copy-legacy-layouts)
 endif()
 
-add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY IS_FRAGILE
+add_swift_target_library(swiftCxx STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY IS_FRAGILE
     CxxConvertibleToCollection.swift
     CxxDictionary.swift
     CxxPair.swift