[cxx-interop] Declare CxxStdlib dependency on _Builtin_float

This fixes incremental builds of the overlay.

The CxxStdlib overlay re-exports the entire C++ standard library, which has headers that transitively include Clang builtin headers, bringing in the _Builtin_float module.

rdar://140036608

Author: egorzhdan

stdlib/public/Cxx/std/CMakeLists.txt

@@ -28,6 +28,11 @@ if(SWIFT_BUILD_STDLIB AND SWIFT_BUILD_SDK_OVERLAY)
   set(swift_cxxstdlib_darwin_dependencies Darwin)
 endif()
 
+set(swift_cxxstdlib_dependencies)
+if(NOT DEFINED SWIFT_BUILD_CLANG_OVERLAYS_SKIP_BUILTIN_FLOAT OR NOT SWIFT_BUILD_CLANG_OVERLAYS_SKIP_BUILTIN_FLOAT)
+  set(swift_cxxstdlib_dependencies _Builtin_float)
+endif()
+
 #
 # C++ Standard Library Overlay.
 #
@@ -38,7 +43,7 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_O
     Chrono.swift
     String.swift
 
-    SWIFT_MODULE_DEPENDS Cxx
+    SWIFT_MODULE_DEPENDS Cxx ${swift_cxxstdlib_dependencies}
     SWIFT_MODULE_DEPENDS_IOS ${swift_cxxstdlib_darwin_dependencies}
     SWIFT_MODULE_DEPENDS_OSX ${swift_cxxstdlib_darwin_dependencies}
     SWIFT_MODULE_DEPENDS_TVOS ${swift_cxxstdlib_darwin_dependencies}