[clangd] Fix isInsideMainFile to be aware of preamble.

clang-tools-extra/clangd/SourceCode.cpp

if (SM.isMacroArgExpansion(FileRange.getBegin())) {
       FileRange = unionTokenRange(ExpansionRangeForBegin, ExpansionRangeForEnd,
                                   SM, LangOpts);
     }
   }
   return FileRange;
 }
 
 bool isInsideMainFile(SourceLocation Loc, const SourceManager &SM) {
-  return Loc.isValid() && SM.isWrittenInMainFile(SM.getExpansionLoc(Loc));
+  if (!Loc.isValid())
+    return false;
+  FileID FID = SM.getFileID(SM.getExpansionLoc(Loc));
+  return FID == SM.getMainFileID() || FID == SM.getPreambleFileID();
 }
 
 llvm::Optional<SourceRange> toHalfOpenFileRange(const SourceManager &SM,
                                                 const LangOptions &LangOpts,
                                                 SourceRange R) {
   SourceRange R1 = getTokenFileRange(R.getBegin(), SM, LangOpts);
   if (!isValidFileRange(SM, R1))
     return llvm::None;

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

TU.HeaderCode = R"cpp(
     #define DEFINE_CLASS(X) class X {};
     #define DEFINE_YY DEFINE_CLASS(YY)
 
     class Header1 {};
     DEFINE_CLASS(Header2)
     class Header {};
   )cpp";
   TU.Code = R"cpp(
+    #define DEFINE_MAIN4 class Main4{};
     class Main1 {};
     DEFINE_CLASS(Main2)
     DEFINE_YY
     class Main {};
+    DEFINE_MAIN4
   )cpp";
   TU.ExtraArgs.push_back("-DHeader=Header3");
   TU.ExtraArgs.push_back("-DMain=Main3");
   auto AST = TU.build();
   const auto& SM = AST.getSourceManager();
   auto DeclLoc = [&AST](llvm::StringRef Name) {
     return findDecl(AST, Name).getLocation();
   };
   for (const auto *HeaderDecl : {"Header1", "Header2", "Header3"})
-    EXPECT_FALSE(isInsideMainFile(DeclLoc(HeaderDecl), SM));
+    EXPECT_FALSE(isInsideMainFile(DeclLoc(HeaderDecl), SM)) << HeaderDecl;
 
-  for (const auto *MainDecl : {"Main1", "Main2", "Main3", "YY"})
-    EXPECT_TRUE(isInsideMainFile(DeclLoc(MainDecl), SM));
+  for (const auto *MainDecl : {"Main1", "Main2", "Main3", "Main4", "YY"})
+    EXPECT_TRUE(isInsideMainFile(DeclLoc(MainDecl), SM)) << MainDecl;
+
+  // Main4 is *spelled* in the preamble, but in the main-file part of it.
+  EXPECT_TRUE(isInsideMainFile(SM.getSpellingLoc(DeclLoc("Main4")), SM));
 }
 
 // Test for functions toHalfOpenFileRange and getHalfOpenFileRange
 TEST(SourceCodeTests, HalfOpenFileRange) {
   // Each marked range should be the file range of the decl with the same name
   // and each name should be unique.
   Annotations Test(R"cpp(
     #define FOO(X, Y) int Y = ++X