[flang] Fix handling of space between # and name in preprocessor stringification

[klausler]

Summary

When preprocessing "# ARG" in function-like macro expansion,
the preprocessor needs to pop the previously-pushed '#' token
from the end of the resulting token sequence after detecting the
argument name. The code to do this was just wrong in a couple of
ways.

flang/lib/Parser/preprocessor.cpp

if (bytes == 2 && token[0] == '~') { // argument substitution
       std::size_t prev{j};
       while (prev > 0 && replacement_.TokenAt(prev - 1).IsBlank()) {
         --prev;
       }
       if (prev > 0 && replacement_.TokenAt(prev - 1).size() == 1 &&
           replacement_.TokenAt(prev - 1)[0] ==
               '#') { // stringify argument without macro replacement
         std::size_t resultSize{result.SizeInTokens()};
-        while (resultSize > 0 && result.TokenAt(resultSize - 1).empty()) {
+        while (resultSize > 0 && result.TokenAt(resultSize - 1).IsBlank()) {
           result.pop_back();
+          --resultSize;
         }
         CHECK(resultSize > 0 &&
             result.TokenAt(resultSize - 1) == replacement_.TokenAt(prev - 1));
         result.pop_back();
         result.Put(Stringify(args[index], prescanner.allSources()));
       } else {
         const TokenSequence *arg{&args[index]};
         std::optional<TokenSequence> replaced;

flang/test/Preprocessing/pp045.F

+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: n = "foobar" // "baz"
+* # stringizing works in FLM
+#define STR(x) # x
+#define MAC(a,b) STR(a ## b)
+      program main
+      character(6) n
+      n = MAC(foo, bar) // STR( baz )
+      end

flang/test/Preprocessing/pp131.F90

+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: n = "foobar" // "baz"
+! # stringizing works in FLM
+#define STR(x) # x
+#define MAC(a,b) STR(a ## b)
+program main
+  character(6) n
+  n = MAC(foo, bar) // STR( baz )
+end