[Lex] Consider a PCH header-guarded even with #endif truncated

[sammccall]

Summary

This seems to be a more useful behavior for tools that use preambles.
I believe it doesn't affect real compiles: the PCH is only included once
when used, and recursive inclusion of the main-file *within* the PCH
isn't supported in any case.

clang-tools-extra/clangd/unittests/ParsedASTTests.cpp

 TEST(ParsedASTTest, HeaderGuards) {
   TestTU TU;
   TU.ImplicitHeaderGuard = false;
 
   TU.Code = ";";
   EXPECT_FALSE(mainIsGuarded(TU.build()));
 
   TU.Code = guard(";");
-  EXPECT_FALSE(mainIsGuarded(TU.build())); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(TU.build()));
 
   TU.Code = once(";");
   EXPECT_TRUE(mainIsGuarded(TU.build()));
 
   TU.Code = R"cpp(
     ;
     #pragma once
   )cpp";
TU.Code = R"cpp(
     #ifndef GUARD
     #define GUARD
     ;
     #include "self.h"
     #endif
   )cpp";
   AST = TU.build();
   EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
-  EXPECT_FALSE(mainIsGuarded(AST)); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(AST));
 
   // Guarded too late...
   TU.Code = R"cpp(
     #include "self.h" // error-ok
     #ifndef GUARD
     #define GUARD
     ;
     #endif
TEST(ParsedASTTest, HeaderGuardsImplIface) {
 
   // Editing the interface file, which is include guarded (easy case).
   // We mostly get this right via PP if we don't recognize the include guard.
   TU.Filename = "iface.h";
   TU.Code = guard(Interface);
   TU.AdditionalFiles = {{"impl.h", Implementation}};
   auto AST = TU.build();
   EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
-  EXPECT_FALSE(mainIsGuarded(AST)); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(AST));
   // Slightly harder: the `#pragma once` is part of the preamble, and we
   // need to transfer it to the main file's HeaderFileInfo.
   TU.Code = once(Interface);
   AST = TU.build();
   EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
   EXPECT_TRUE(mainIsGuarded(AST));
 
   // Editing the implementation file, which is not include guarded.

clang/lib/Lex/Lexer.cpp

if (isLexingRawMode()) {
     Result.startToken();
     BufferPtr = BufferEnd;
     FormTokenWithChars(Result, BufferEnd, tok::eof);
     return true;
   }
 
   if (PP->isRecordingPreamble() && PP->isInPrimaryFile()) {
     PP->setRecordedPreambleConditionalStack(ConditionalStack);
+    // If the preamble cuts off the end of a header guard, consider it guarded.
+    // The guard is valid for the preamble content itself, and for tools the
+    // most useful answer is "yes, this file has a header guard".
+    if (!ConditionalStack.empty())
+      MIOpt.ExitTopLevelConditional();
     ConditionalStack.clear();
   }
 
   // Issue diagnostics for unterminated #if and missing newline.
 
   // If we are in a #if directive, emit an error.
   while (!ConditionalStack.empty()) {
     if (PP->getCodeCompletionFileLoc() != FileLoc)