build: correct the llbuild build when vendored

The value needs to be pushed through the cache as there is no control
over the flags for the sub-build and `set` is scoped to the current
directory.

Author: compnerd

CMakeLists.txt

@@ -66,7 +66,7 @@ if(NOT LLBuild_FOUND)
     find_package(LLBuild REQUIRED)
   else()
     message("-- Vending swift-llbuild")
-    set(LLBUILD_SUPPORT_BINDINGS Swift)
+    set(LLBUILD_SUPPORT_BINDINGS Swift CACHE STRING "")
     FetchContent_Declare(LLBuild
       GIT_REPOSITORY https://github.com/apple/swift-llbuild
       GIT_TAG main)