Fix issue SR-9482.

Solution based on the outline in the issue description.

Also fixes the corresponding rdar://problem/46644027.

Author: varungandhi-apple

lib/ClangImporter/ImportName.cpp

@@ -1729,9 +1729,14 @@ ImportedName NameImporter::importNameImpl(const clang::NamedDecl *D,
 
 /// Returns true if it is expected that the macro is ignored.
 static bool shouldIgnoreMacro(StringRef name, const clang::MacroInfo *macro) {
-  // Ignore include guards.
-  if (macro->isUsedForHeaderGuard())
-    return true;
+  // Ignore include guards. Try not to ignore definitions of useful constants,
+  // which may end up looking like include guards.
+  if (macro->isUsedForHeaderGuard() && macro->getNumTokens() == 1) {
+    auto tok = macro->tokens()[0];
+    if (tok.isLiteral()
+        && StringRef(tok.getLiteralData(), tok.getLength()) == "1")
+      return true;
+  }
 
   // If there are no tokens, there is nothing to convert.
   if (macro->tokens_empty())

test/ClangImporter/macros.swift

@@ -179,5 +179,5 @@ func testNulls() {
 
 func testHeaderGuard() {
   _ = IS_HEADER_GUARD // expected-error {{use of unresolved identifier 'IS_HEADER_GUARD'}}
-  _ = LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT // expected-error {{use of unresolved identifier 'LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT'}}
+  _ = LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT
 }