[Serialization] Minor improvements to doc from PR comments

Author: xymus

include/swift/AST/Module.h

@@ -545,7 +545,7 @@ class ModuleDecl
   }
 
   /// Returns true if the module was rebuilt from a module interface instead
-  /// of being build from the full source.
+  /// of being built from the full source.
   bool isBuiltFromInterface() const {
     return Bits.ModuleDecl.IsBuiltFromInterface;
   }

include/swift/ClangImporter/ClangImporter.h

@@ -220,8 +220,8 @@ class ClangImporter final : public ClangModuleLoader {
   /// \param path A sequence of (identifier, location) pairs that denote
   /// the dotted module name to load, e.g., AppKit.NSWindow.
   ///
-  /// \param AllowMemoryCache Enables preserving the loaded module in the
-  /// in-memory cache for the next loading attempt.
+  /// \param AllowMemoryCache Affects only loading serialized Swift modules,
+  /// this parameter has no effect in the ClangImporter.
   ///
   /// \returns the module referenced, if it could be loaded. Otherwise,
   /// emits a diagnostic and returns NULL.

lib/FrontendTool/FrontendTool.cpp

@@ -430,8 +430,10 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
       Invocation.getFrontendOptions().RequestedAction !=
         FrontendOptions::ActionType::TypecheckModuleFromInterface &&
       !Invocation.getFrontendOptions().ExplicitInterfaceBuild) {
-      IgnoreAdjacentModules = Instance.getASTContext().IgnoreAdjacentModules;
-    }
+    // Only pass down IgnoreAdjacentModules when the parent instance has an
+    // ASTContext.
+    IgnoreAdjacentModules = Instance.getASTContext().IgnoreAdjacentModules;
+  }
 
   // If an explicit interface build was requested, bypass the creation of a new
   // sub-instance from the interface which will build it in a separate thread,