[cxx-interop] Do not add a dependency on clang to CxxStdlib

Cxx & CxxStdlib modules are Swift-only, they do not require invoking clang directly.

When building with `SWIFT_INCLUDE_TOOLS=NO`, Clang is not available as a CMake target (see `swift_common_standalone_build_config`).

rdar://107780733

Author: egorzhdan

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1678,6 +1678,7 @@ function(add_swift_target_library name)
         IS_SDK_OVERLAY
         IS_STDLIB
         IS_STDLIB_CORE
+        IS_SWIFT_ONLY
         NOSWIFTRT
         OBJECT_LIBRARY
         SHARED
@@ -1814,7 +1815,7 @@ function(add_swift_target_library name)
   endif()
 
   if(NOT SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER AND NOT BUILD_STANDALONE AND
-     NOT SWIFT_PREBUILT_CLANG)
+     NOT SWIFT_PREBUILT_CLANG AND NOT SWIFTLIB_IS_SWIFT_ONLY)
     list(APPEND SWIFTLIB_DEPENDS clang)
   endif()
 

stdlib/public/Cxx/CMakeLists.txt

@@ -3,7 +3,7 @@ if("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WINDOWS")
   set(SWIFT_CXX_LIBRARY_KIND SHARED)
 endif()
 
-add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDLIB
+add_swift_target_library(swiftCxx ${SWIFT_CXX_LIBRARY_KIND} NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY
     CxxConvertibleToCollection.swift
     CxxDictionary.swift
     CxxPair.swift

stdlib/public/Cxx/std/CMakeLists.txt

@@ -127,7 +127,7 @@ add_dependencies(sdk-overlay libstdcxx-modulemap)
 #
 # C++ Standard Library Overlay.
 #
-add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB
+add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY
     std.swift
     String.swift