Address review comments except for prebuilt module dependencies

Author: cyndyishida

clang/include/clang/Tooling/DependencyScanning/ModuleDepCollector.h

@@ -114,8 +114,12 @@ struct ModuleDeps {
   /// Whether this is a "system" module.
   bool IsSystem;
 
-  /// Whether this is a module where it's dependencies resolve within the
-  /// sysroot.
+  /// Whether this module is fully composed of file & module inputs from the
+  /// sysroot. External paths, as opposed to virtual file paths, are always used
+  /// for computing this value.
+  ///
+  /// This attribute is useful for identifying modules that are unlikely to
+  /// change under an active development and build cycle.
   bool IsInSysroot;
 
   /// The path to the modulemap file which defines this module.

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -748,11 +748,12 @@ ModuleDepCollectorPP::handleTopLevelModule(const Module *M) {
   MDC.ScanInstance.getASTReader()->visitInputFileInfos(
       *MF, /*IncludeSystem=*/true,
       [&](const serialization::InputFileInfo &IFI, bool IsSystem) {
-        auto FullFilePath = ASTReader::ResolveImportedPath(
-            PathBuf, IFI.UnresolvedImportedFilename, MF->BaseDirectory);
-        if (MD.IsInSysroot)
+        if (MD.IsInSysroot) {
+          auto FullFilePath = ASTReader::ResolveImportedPath(
+              PathBuf, IFI.UnresolvedImportedFilename, MF->BaseDirectory);
           MD.IsInSysroot = FullFilePath->starts_with(CurrSysroot);
-        PathBuf.resize_for_overwrite(256);
+          PathBuf.resize_for_overwrite(256);
+        }
         if (!(IFI.TopLevel && IFI.ModuleMap))
           return;
         if (IFI.UnresolvedImportedFilenameAsRequested.ends_with(