FindMPI: make sure computed link flags are not de-duplicated

In commit f7eaa34 (FindMPI: Store imported target link flags as a
list instead of a string, 2019-06-14, v3.15.0-rc2~2^2) we used
`separate_arguments` to parse the extracted link flags and add them to
`INTERFACE_LINK_LIBRARIES`.  That property is not meant for link flags
and CMake may de-duplicate them.  This is particularly problematic for
flags like `-Wl,-rpath -Wl,/path1 -Wl,-rpath -Wl,/path2`.

In commit 39c572c (FindMPI: Updated to use INTERFACE_LINK_OPTIONS,
2019-06-24) we moved the parsed flags over to `INTERFACE_LINK_OPTIONS`,
but that may still perform de-duplication.  Avoid the parsing and
de-duplication of flags by passing the original string via `SHELL:`
instead.

Fixes: #19516

Author: Robert Maynard

Modules/FindMPI.cmake

@@ -1143,9 +1143,7 @@ macro(_MPI_create_imported_target LANG)
   set_property(TARGET MPI::MPI_${LANG} PROPERTY INTERFACE_COMPILE_DEFINITIONS "${MPI_${LANG}_COMPILE_DEFINITIONS}")
 
   if(MPI_${LANG}_LINK_FLAGS)
-    separate_arguments(_MPI_${LANG}_LINK_FLAGS NATIVE_COMMAND "${MPI_${LANG}_LINK_FLAGS}")
-    set_property(TARGET MPI::MPI_${LANG} PROPERTY INTERFACE_LINK_OPTIONS "${_MPI_${LANG}_LINK_FLAGS}")
-    unset(_MPI_${LANG}_LINK_FLAGS)
+    set_property(TARGET MPI::MPI_${LANG} PROPERTY INTERFACE_LINK_OPTIONS "SHELL:${MPI_${LANG}_LINK_FLAGS}")
   endif()
   # If the compiler links MPI implicitly, no libraries will be found as they're contained within
   # CMAKE_<LANG>_IMPLICIT_LINK_LIBRARIES already.