[cmake] Build Swift host libraries with -DCLANG_BUILD_STATIC on Windows

By default (`CLANG_BUILD_STATIC` and `CLANG_EXPORTS` not defined) some
Clang visibility macros expand to `__declspec(dllimport)`. This change
ensures that the compiled visibilities for a Swift host library that
statically depends on Clang match up with the compiled visibilities of a
consumer that also statically depends on Clang, such as LLDB.

Author: AnthonyLatsis

cmake/modules/AddSwift.cmake

@@ -269,6 +269,17 @@ function(_add_host_variant_c_compile_flags target)
       target_compile_options(${target} PRIVATE
         $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-U_DEBUG>)
     endif()
+
+    # Swift host libraries are always linked with Clang libraries statically,
+    # so turn off Clang visibility macros for both the library and its
+    # consumers.
+    target_compile_definitions(${target} PUBLIC
+      $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:CLANG_BUILD_STATIC>)
+    if(TARGET "obj.${target}")
+      target_compile_definitions("obj.${target}" PUBLIC
+        $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:CLANG_BUILD_STATIC>)
+    endif()
+  endif()
   endif()
 
   if(SWIFT_HOST_VARIANT_SDK STREQUAL "ANDROID")