[libcxx] Add a dependency on unwind in cxx_static, if LIBCXXABI_USE_LLVM_UNWINDER is set

Even if building cxx_static in itself doesn't actually link in the
requested unwinder, add a synthetic dependency so that building
cxx_static makes sure that the unwinder that was requested to be used
also gets built.

This makes sure that tests (when run with just a plain "ninja check-cxx")
actually use the newly built unwinder, as intended.

Differential Revision: https://reviews.llvm.org/D113467

libcxx/src/CMakeLists.txt

@@ -286,6 +286,13 @@ if (LIBCXX_ENABLE_STATIC)
   cxx_add_common_build_flags(cxx_static)
   cxx_set_common_defines(cxx_static)
 
+  if (LIBCXXABI_USE_LLVM_UNWINDER)
+    # If we intend to use the just-built unwinder, add a dependency so that it
+    # gets built, even if we technically aren't going to link it in at this
+    # stage.
+    add_dependencies(cxx_static unwind)
+  endif()
+
   if (LIBCXX_HERMETIC_STATIC_LIBRARY)
     # If the hermetic library doesn't define the operator new/delete functions
     # then its code shouldn't declare them with hidden visibility.  They might