[clang][index] Handle undefined function-like macros in single file parse mode (llvm#135054)

The single file parse mode is supposed to enter both branches of an
`#if` directive whenever the condition contains undefined identifiers.
This patch adds support for undefined function-like macros, where we
would previously emit an error that doesn't make sense from end-user
perspective.

(I discovered this while working on a very similar feature that parses
single module only and doesn't enter either `#if` branch when the
condition contains undefined identifiers.)

Author: jansvoboda11

clang/lib/Lex/PPExpressions.cpp

@@ -26,6 +26,7 @@
 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/PPCallbacks.h"
 #include "clang/Lex/Preprocessor.h"
+#include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Lex/Token.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/STLExtras.h"
@@ -592,6 +593,15 @@ static bool EvaluateDirectiveSubExpr(PPValue &LHS, unsigned MinPrec,
                                      Token &PeekTok, bool ValueLive,
                                      bool &IncludedUndefinedIds,
                                      Preprocessor &PP) {
+  if (PP.getPreprocessorOpts().SingleFileParseMode && IncludedUndefinedIds) {
+    // The single-file parse mode behavior kicks in as soon as single identifier
+    // is undefined. If we've already seen one, there's no point in continuing
+    // with the rest of the expression. Besides saving work, this also prevents
+    // calling undefined function-like macros.
+    PP.DiscardUntilEndOfDirective(PeekTok);
+    return true;
+  }
+
   unsigned PeekPrec = getPrecedence(PeekTok.getKind());
   // If this token isn't valid, report the error.
   if (PeekPrec == ~0U) {

clang/test/Index/single-file-parse-undefined-function-like-macro.c

@@ -0,0 +1,15 @@
+// RUN: split-file %s %t
+// RUN: c-index-test -single-file-parse %t/tu.c 2>&1 | FileCheck %t/tu.c
+
+//--- header.h
+#define FUNCTION_LIKE_MACRO() 1
+//--- tu.c
+#include "header.h"
+// CHECK-NOT: tu.c:[[@LINE+1]]:5: error: function-like macro 'FUNCTION_LIKE_MACRO' is not defined
+#if FUNCTION_LIKE_MACRO()
+// CHECK: tu.c:[[@LINE+1]]:5: FunctionDecl=then_fn
+int then_fn();
+#else
+// CHECK: tu.c:[[@LINE+1]]:5: FunctionDecl=else_fn
+int else_fn();
+#endif