[Flang][OpenMP] Prevent ICE for certain constructs in unnamed programs (#73938)

This patch fixes #72748 by modifying the processing of program units to
search for a symbol to which OpenMP REQUIRES clauses can bind to. Rather
than picking up the first PFT node with a source reference and getting
its associated scope, it picks up the last one.

This avoids using the source from the first specification construct of
a nameless program, which can sometimes not be associated to any scope,
causing an ICE due to an invalid source location.

Author: skatrak

flang/lib/Semantics/resolve-directives.cpp

@@ -26,7 +26,7 @@
 template <typename T>
 static Fortran::semantics::Scope *GetScope(
     Fortran::semantics::SemanticsContext &context, const T &x) {
-  std::optional<Fortran::parser::CharBlock> source{GetSource(x)};
+  std::optional<Fortran::parser::CharBlock> source{GetLastSource(x)};
   return source ? &context.FindScope(*source) : nullptr;
 }
 

flang/test/Semantics/OpenMP/struct.f90

@@ -0,0 +1,7 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
+! Check OpenMP compatibility with the DEC STRUCTURE extension
+
+structure /s/
+end structure
+
+end