[flang] Fix dangling pointer in LabelEnforce

`DirectiveStructureChecker` was passing in a pointer to a temporary
string for the `construct` argument to the constructor for `LabelEnforce`.
The `LabelEnforce` object had a lifetime longer than the temporary,
resulting in accessing a dangling pointer when emitting an error message
for `omp-parallell01.f90`.

The fix is to make the lifetime of the temporary as long as the lifetime
of the `LabelEnforce` object.

Differential Revision: https://reviews.llvm.org/D94618

flang/lib/Semantics/check-directive-structure.h

@@ -280,9 +280,9 @@ void DirectiveStructureChecker<D, C, PC, ClauseEnumSize>::CheckNoBranching(
       context_, directiveSource, directive, ContextDirectiveAsFortran()};
   parser::Walk(block, noBranchingEnforce);
 
+  auto construct{parser::ToUpperCaseLetters(getDirectiveName(directive).str())};
   LabelEnforce directiveLabelEnforce{context_, noBranchingEnforce.labels(),
-      directiveSource,
-      parser::ToUpperCaseLetters(getDirectiveName(directive).str()).c_str()};
+      directiveSource, construct.c_str()};
   parser::Walk(block, directiveLabelEnforce);
 }