[flang][OpenMP] Rename variable block to body, NFC (#156702)

`block` is a global variable that represents the Block parser, so avoid
using local variables with the same name.

Author: kparzysz

flang/lib/Parser/openmp-parsers.cpp

@@ -1289,9 +1289,9 @@ struct StrictlyStructuredBlockParser {
     if (lookAhead(skipStuffBeforeStatement >> "BLOCK"_tok).Parse(state)) {
       if (auto epc{Parser<ExecutionPartConstruct>{}.Parse(state)}) {
         if (IsFortranBlockConstruct(*epc)) {
-          Block block;
-          block.emplace_back(std::move(*epc));
-          return std::move(block);
+          Block body;
+          body.emplace_back(std::move(*epc));
+          return std::move(body);
         }
       }
     }