[flang] Handle "defined" in macro expansions (#114844)

The preprocessor implements "defined(X)" and "defined X" in if/elif
directive expressions in such a way that they only work at the top
level, not when they appear in macro expansions. Fix that, which is a
little tricky due to the need to detect the "defined" keyword before
applying any macro expansion to its argument, and add a bunch of tests.

Fixes https://github.com/llvm/llvm-project/issues/114064.

1 files changed, 10 insertions, 0 deletions

diff --git a/flang/lib/Parser/token-sequence.cpp b/flang/lib/Parser/token-sequence.cpp
index c73bca1..cdbe89b 100644
--- a/flang/lib/Parser/token-sequence.cpp
+++ b/flang/lib/Parser/token-sequence.cpp
@@ -61,6 +61,16 @@ std::size_t TokenSequence::SkipBlanks(std::size_t at) const {
   return tokens; // even if at > tokens
 }
 
+std::optional<std::size_t> TokenSequence::SkipBlanksBackwards(
+    std::size_t at) const {
+  while (at-- > 0) {
+    if (!TokenAt(at).IsBlank()) {
+      return at;
+    }
+  }
+  return std::nullopt;
+}
+
 // C-style /*comments*/ are removed from preprocessing directive
 // token sequences by the prescanner, but not C++ or Fortran
 // free-form line-ending comments (//...  and !...) because