[sourcekit] Change getASTManager() to return shared_ptr

Addressing review feedback: this avoids calling shared_from_this() from
outside the implementation.  Note: it is not possible to use private
inheritance of enable_shared_from_this to prevent this issue in general,
because enabled_shared_from_this relies on the shared_ptr constructor
being able to detect that the type has this inheritance, which means it
must be public.

tools/SourceKit/lib/SwiftLang/SwiftIndexing.cpp

@@ -251,10 +251,10 @@ void SwiftLangSupport::indexSource(StringRef InputFile,
   CompilerInvocation Invocation;
   bool Failed = true;
   if (IsModuleIndexing) {
-    Failed = getASTManager().initCompilerInvocationNoInputs(
+    Failed = getASTManager()->initCompilerInvocationNoInputs(
         Invocation, Args, CI.getDiags(), Error);
   } else {
-    Failed = getASTManager().initCompilerInvocation(
+    Failed = getASTManager()->initCompilerInvocation(
         Invocation, Args, CI.getDiags(), InputFile, Error);
   }
   if (Failed) {