[flang] Fix #else with trailing text (llvm#138045)

Fixed the issue, where the extra text on #else line (' Z' in the example
below) caused the data from the "else" clause to be processed together
with the data of "then" clause.
```
#ifndef XYZ42 
      PARAMETER(A=2)
#else Z
      PARAMETER(A=3)
#endif
      end
```

Author: eugeneepshteyn

flang/lib/Parser/preprocessor.cpp

@@ -684,7 +684,8 @@ void Preprocessor::Directive(const TokenSequence &dir, Prescanner &prescanner) {
             dir.GetIntervalProvenanceRange(j, tokens - j),
             "#else: excess tokens at end of directive"_port_en_US);
       }
-    } else if (ifStack_.empty()) {
+    }
+    if (ifStack_.empty()) {
       prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
           "#else: not nested within #if, #ifdef, or #ifndef"_err_en_US);
     } else if (ifStack_.top() != CanDeadElseAppear::Yes) {

flang/test/Preprocessing/pp048.F

@@ -0,0 +1,11 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+#ifndef XYZ42 
+      PARAMETER(A=2)
+#else Z
+      PARAMETER(A=3)
+#endif
+! Ensure that "PARAMETER(A" is printed only once
+! CHECK: PARAMETER(A
+! CHECK-NOT: PARAMETER(A
+      end
+