[Templight] Template Instantiation Observer

This patch adds a base-class called TemplateInstantiationObserver which gets
notified whenever a template instantiation is entered or exited during
semantic analysis. This is a base class used to implement the template
profiling and debugging tool called
Templight (https://github.com/mikael-s-persson/templight).

The patch also makes a few more changes:

* ActiveTemplateInstantiation class is moved out of the Sema class (so it can be used with inclusion of Sema.h).
* CreateFrontendAction function in front-end utilities is given external linkage (not longer a hidden static function).
* TemplateInstObserverChain data member added to Sema class to hold the list of template-inst observers.
* Notifications to the template-inst observer are added at the key places where templates are instantiated.

Patch by: Abel Sinkovics!

Differential Revision: https://reviews.llvm.org/D5767

llvm-svn: 324808
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 45fd2d3..65183b4 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -37,6 +37,7 @@
 #include "clang/Sema/SemaConsumer.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/TemplateDeduction.h"
+#include "clang/Sema/TemplateInstCallback.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallSet.h"
 using namespace clang;