Identify constant definitions that look like include guard definitions

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

@@ -176,3 +176,8 @@ func testNulls() {
   let _: Int = DEPRECATED_ONE // expected-error {{use of unresolved identifier 'DEPRECATED_ONE'}}
   let _: Int = OKAY_TYPED_ONE // expected-error {{cannot convert value of type 'okay_t' (aka 'UInt32') to specified type 'Int'}}
 }
+
+func testHeaderGuard() {
+  _ = IS_HEADER_GUARD // expected-error {{use of unresolved identifier 'IS_HEADER_GUARD'}}
+  _ = LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT
+}

test/Inputs/clang-importer-sdk/usr/include/header_guard.h

@@ -0,0 +1,3 @@
+#ifndef IS_HEADER_GUARD
+#define IS_HEADER_GUARD
+#endif

test/Inputs/clang-importer-sdk/usr/include/macros.h

@@ -1,5 +1,7 @@
 #include <macros_impl.h>
 #include <macros_private_impl.h>
+#include <header_guard.h>
+#include <not_a_header_guard.h>
 
 // Get Clang's NULL.
 #include <stddef.h>

test/Inputs/clang-importer-sdk/usr/include/not_a_header_guard.h

@@ -0,0 +1,3 @@
+#ifndef LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT
+#define LOOKS_LIKE_HEADER_GUARD_BUT_IS_USEFUL_CONSTANT 0xcafe
+#endif