CheckIPOSupported: Tolerate backslashes in output of failed checks

Avoid passing the output of our `try_compile` through a macro argument.
Macro invocations re-parse their arguments and so should not be given
arbitrary content that may contain backslashes and such.

Instead pass a simple message.  This is also more readable in the case
that the macro generates a message itself.  After the macro returns,
report the real output directly to the variable named by our caller.
Also record the output in a log file as other checks do on failure.

Fixes: #18244

Modules/CheckIPOSupported.cmake

@@ -126,7 +126,13 @@ macro(_ipo_run_language_check language)
   )
 
   if(NOT result)
-    _ipo_not_supported("${output}")
+    file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+      "${language} compiler IPO check failed with the following output:\n"
+      "${output}\n")
+    _ipo_not_supported("check failed to compile")
+    if(X_OUTPUT)
+      set("${X_OUTPUT}" "${output}" PARENT_SCOPE)
+    endif()
     return()
   endif()
 endmacro()