[flang][OpenMP] Detect BLOCK construct through lookahead (llvm#150617)

Avoid parsing the entire ExecutionPartConstruct in either the strictly-
or the loosely-structured block parser only to discard it when it's not
BLOCK (or is BLOCK) respectively. Doing so was not incorrct, but in
pathological cases (like Fujitsu 0981_0034) the recursive parsing can
take a very long time.

Instead, detect the presence of BLOCK first (via a simple lookahead),
and fail immediately if necessary.

Author: kparzysz

flang/lib/Parser/openmp-parsers.cpp

@@ -1222,11 +1222,14 @@ struct StrictlyStructuredBlockParser {
   using resultType = Block;
 
   std::optional<resultType> Parse(ParseState &state) const {
-    if (auto epc{Parser<ExecutionPartConstruct>{}.Parse(state)}) {
-      if (IsFortranBlockConstruct(*epc)) {
-        Block block;
-        block.emplace_back(std::move(*epc));
-        return std::move(block);
+    // Detect BLOCK construct without parsing the entire thing.
+    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);
+        }
       }
     }
     return std::nullopt;
@@ -1237,6 +1240,10 @@ struct LooselyStructuredBlockParser {
   using resultType = Block;
 
   std::optional<resultType> Parse(ParseState &state) const {
+    // Detect BLOCK construct without parsing the entire thing.
+    if (lookAhead(skipStuffBeforeStatement >> "BLOCK"_tok).Parse(state)) {
+      return std::nullopt;
+    }
     Block body;
     if (auto epc{attempt(Parser<ExecutionPartConstruct>{}).Parse(state)}) {
       if (!IsFortranBlockConstruct(*epc)) {