build-script: always rebuild XCTest on non-Darwin

This should resolve SR-8826!  Ideally, we would be using CMake to tie together
all the dependent packages and get proper dependency tracking.  Currently,
change to the swift runtime do not get tracked properly, and can result in
undefined references to symbols.

Author: compnerd

utils/build-script-impl

@@ -2487,6 +2487,11 @@ for host in "${ALL_HOSTS[@]}"; do
                   continue
                 ;;
                 *)
+                  # FIXME: Always re-build XCTest on non-darwin platforms.
+                  # Remove this when products build in the CMake system.
+                  echo "Cleaning the XCTest build directory"
+                  call rm -rf "${XCTEST_BUILD_DIR}"
+
                   cmake_options=(
                     ${cmake_options[@]}
                     -DCMAKE_BUILD_TYPE:STRING="${LIBDISPATCH_BUILD_TYPE}"