[build] Stop assuming llvm is always the top-level in unified build

Even with unified build, llvm is not always the top-level project but it
can be a part of a larger build. (e.g. [^1]) In that case,
`CMAKE_BINARY_DIR` is not the binary directory of llvm but the binary
directory of the top-level project. This patch fixes the issue by using
`LLVM_BINARY_DIR` instead.

[^1]: https://github.com/ChromeDevTools/devtools-frontend/blob/9b4b9070790196d78489756c515c528a0fe7ac00/extensions/cxx_debugging/CMakeLists.txt#L105

Author: kateinoigakukun

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -276,7 +276,7 @@ macro(swift_common_unified_build_config product)
   set(LLVM_CMAKE_DIR "${CMAKE_SOURCE_DIR}/cmake/modules")
   set(CLANG_INCLUDE_DIRS
     "${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/include"
-    "${CMAKE_BINARY_DIR}/tools/clang/include")
+    "${LLVM_BINARY_DIR}/tools/clang/include")
 
   include_directories(${CLANG_INCLUDE_DIRS})