[flang] Fix dangling pointer in LabelEnforce

[tskeith]

Summary

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.

[sameeranjoshi]

Thank you for the patch.
I did get an email from the bot with a weird text Control flow escapes from \xcb\x1c\xda\xff\xff\nexpect at 16: Control flow escapes from PARALLEL\n'
I had a build of gcc 9.2 and the tried to check with the recent pull from main and all tests seem to work.
It was pointed[1] by @awarzynski that the issue was specifically due to clang-10.

Thank you again.
[1] https://reviews.llvm.org/D93447#2498085

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

 template <typename D, typename C, typename PC, std::size_t ClauseEnumSize>
 void DirectiveStructureChecker<D, C, PC, ClauseEnumSize>::CheckNoBranching(
     const parser::Block &block, D directive,
     const parser::CharBlock &directiveSource) {
   NoBranchingEnforce<D> noBranchingEnforce{
       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);
 }
 
 // Check that only clauses included in the given set are present after the given
 // clause.
 template <typename D, typename C, typename PC, std::size_t ClauseEnumSize>
 void DirectiveStructureChecker<D, C, PC, ClauseEnumSize>::CheckOnlyAllowedAfter(
     C clause, common::EnumSet<C, ClauseEnumSize> set) {