[libcxx] [test] Fix restoring LLVM_DIR and Clang_DIR (llvm#132838)

In 664f345, a fix was introduced,
attempting to restore LLVM_DIR and Clang_DIR after doing
find_package(Clang).

However, 6775285 added a return if the
clangTidy target wasn't found. If this is hit, we don't restore LLVM_DIR
and Clang_DIR, which causes strange effects if CMake is rerun a second
time.

Move the code for restoring LLVM_DIR and Clang_DIR to directly after the
find_package calls, to make sure they are restored, regardless of the
find_package outcome.

(cherry picked from commit 51bceb4)

Author: mstorsjo

libcxx/test/tools/clang_tidy_checks/CMakeLists.txt

@@ -8,6 +8,10 @@ set(Clang_DIR_SAVE ${Clang_DIR})
 # versions must match. Otherwise there likely will be ODR-violations. This had
 # led to crashes and incorrect output of the clang-tidy based checks.
 find_package(Clang ${CMAKE_CXX_COMPILER_VERSION})
+
+set(LLVM_DIR "${LLVM_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for LLVM." FORCE)
+set(Clang_DIR "${Clang_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for Clang." FORCE)
+
 if(NOT Clang_FOUND)
   message(STATUS "Clang-tidy tests are disabled since the "
                  "Clang development package is unavailable.")
@@ -19,9 +23,6 @@ if(NOT TARGET clangTidy)
   return()
 endif()
 
-set(LLVM_DIR "${LLVM_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for LLVM." FORCE)
-set(Clang_DIR "${Clang_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for Clang." FORCE)
-
 message(STATUS "Found system-installed LLVM ${LLVM_PACKAGE_VERSION} with headers in ${LLVM_INCLUDE_DIRS}")
 
 set(CMAKE_CXX_STANDARD 20)