FindOpenMP: Add support for openmp 5.1 (llvm 17) and 5.2

llvm 17 now defaults to openmp 5.1; see LLVM commit `0c6f2f629cc0`.
It makes FindOpenMP set OPENMP_FOUND and OpenMP_C_SPEC_DATE, but not
OpenMP_C_VERSION, because it doesn't know the version mapping.
Add the spec date to the version mapping for 5.1 and 5.2 to make
things work again.

Modules/FindOpenMP.cmake

@@ -434,6 +434,8 @@ endfunction()
 
 macro(_OPENMP_SET_VERSION_BY_SPEC_DATE LANG)
   set(OpenMP_SPEC_DATE_MAP
+    "202111=5.2"
+    "202011=5.1"
     # Preview versions
     "201611=5.0" # OpenMP 5.0 preview 1
     # Combined versions, 2.5 onwards