[NFC][flang] Fix PushSemantics macro

[luporl]

Summary

Add and use the CONCAT macro to force the expansion of LINE in
PushSemantics body.

[clementval]

What is this trying to fix? Do you have an open issue?

[luporl]

In D153460#4439210, @clementval wrote:

What is this trying to fix? Do you have an open issue?

It's trying to fix the name of the local variable used by PushSemantics macro. There is no open issue, I discovered this while debugging.

pushSemanticsLocalVariable##__LINE__ gets preprocessed to pushSemanticsLocalVariable__LINE__.
This patch fixes this, as it seems the original intent was to append the line number to the variable name.

This would also avoid compilation errors in the unlikely case that PushSemantics is used twice in the same scope.
And if PushSemantics is used in a nested way, this avoids shadowing the variables in the outer scopes, which helps when debugging.

[clementval]

Any way to add a test so we can make sure we have what we want?

flang/lib/Lower/ConvertExpr.cpp

3023–3024

Why do we need two macros here?

[luporl]

In D153460#4442500, @clementval wrote:

Any way to add a test so we can make sure we have what we want?

It would be possible to add a unit test that uses PushSemantics in the ways that currently cause issues, but it would have to be moved to ConvertExpr.h. Would it be ok?

flang/lib/Lower/ConvertExpr.cpp

3023–3024

It's because arguments are not expanded in concatenation. That's why we need to call CONCAT2, before doing the concatenation.
This is explained in more depth here: https://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html.

[clementval]

Ok thanks for the clarification. LGTM

flang/lib/Lower/ConvertExpr.cpp

private:
   fir::FirOpBuilder &builder;
   Fortran::lower::StatementContext &stmtCtx;
   Fortran::lower::SymMap &symMap;
   bool inInitializer = false;
   bool useBoxArg = false; // expression lowered as argument
 };
 } // namespace
 
+#define CONCAT(x, y) CONCAT2(x, y)
+#define CONCAT2(x, y) x##y
+
 // Helper for changing the semantics in a given context. Preserves the current
 // semantics which is resumed when the "push" goes out of scope.
 #define PushSemantics(PushVal)                                                 \
-  [[maybe_unused]] auto pushSemanticsLocalVariable##__LINE__ =                 \
+  [[maybe_unused]] auto CONCAT(pushSemanticsLocalVariable, __LINE__) =         \
       Fortran::common::ScopedSet(semant, PushVal);
 
 static bool isAdjustedArrayElementType(mlir::Type t) {
   return fir::isa_char(t) || fir::isa_derived(t) || t.isa<fir::SequenceType>();
 }
 static bool elementTypeWasAdjusted(mlir::Type t) {
   if (auto ty = t.dyn_cast<fir::ReferenceType>())
     return isAdjustedArrayElementType(ty.getEleTy());