[flang]Fix individual tests with lit when building out of tree

[sameeranjoshi]

Summary

This commit is basically extending @CarolineConcatto work[D81002]

Fix individual check tests with lit when building out-of-tree
check-flang-<folder> was not working.
The CMakeLists.txt was looking for the lit tests in the source directory
instead of the build directory.

Both in-tree and out-of-tree works in my sandbox.

[DavidTruby]

Looks good to me, seems a pretty simple fix so I think you can go ahead and commit it

flang/CMakeLists.txt

 option(FLANG_ENABLE_WERROR "Fail and stop building flang if a warning is triggered."
        "${_FLANG_ENABLE_WERROR_DEFAULT}")
 
 # Check for a standalone build and configure as appropriate from
 # there.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   message("Building Flang as a standalone project.")
   project(Flang)
+  set(FLANG_STANDALONE_BUILD ON)
 
   set(FLANG_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
   if (NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")
     # Assertions follow llvm's configuration.
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()
#LLVM_BUILD_MAIN_SRC_DIR - Path to llvm source when out-of-tree.
   else()
     add_custom_target(check-all DEPENDS check-flang )
   endif()
   if (LLVM_BUILD_DOCS)
     add_custom_target(doxygen ALL)
   endif()
 
 else()
+  set(FLANG_STANDALONE_BUILD OFF)
   option(FLANG_INCLUDE_TESTS
          "Generate build targets for the Flang unit tests."
          ${LLVM_INCLUDE_TESTS})
   set(FLANG_GTEST_AVAIL 1)
 
   set(FLANG_BINARY_DIR ${CMAKE_BINARY_DIR}/tools/flang)
   set(BACKEND_PACKAGE_STRING "${PACKAGE_STRING}")
   if (LINK_WITH_FIR)

flang/test/CMakeLists.txt

 
 add_lit_testsuite(check-flang "Running the Flang regression tests"
   ${CMAKE_CURRENT_BINARY_DIR}
   PARAMS ${FLANG_TEST_PARAMS}
   DEPENDS ${FLANG_TEST_DEPENDS}
 )
 set_target_properties(check-flang PROPERTIES FOLDER "Tests")
 
+# In case of standalone builds.
+if (FLANG_STANDALONE_BUILD)
+  add_lit_testsuites(FLANG ${CMAKE_CURRENT_BINARY_DIR}
+    PARAMS ${FLANG_TEST_PARAMS}
+    DEPENDS ${FLANG_TEST_DEPENDS})
+else()
   add_lit_testsuites(FLANG ${CMAKE_CURRENT_SOURCE_DIR}
     PARAMS ${FLANG_TEST_PARAMS}
     DEPENDS ${FLANG_TEST_DEPENDS})
-
+endif()