[ASTImporter] Support LambdaExprs and improve template support

Also, a number of style and bug fixes was done:

 *  ASTImporterTest: added sanity check for source node
 *  ExternalASTMerger: better lookup for template specializations
 *  ASTImporter: don't add templated declarations into DeclContext
 *  ASTImporter: introduce a helper, ImportTemplateArgumentListInfo getting SourceLocations
 *  ASTImporter: proper set ParmVarDecls for imported FunctionProtoTypeLoc

Differential Revision: https://reviews.llvm.org/D42301

llvm-svn: 323519

clang/test/ASTMerge/function-cpp/Inputs/function-1.cpp

@@ -0,0 +1,8 @@
+
+template<typename T> constexpr T add(T arg1, T arg2) {
+  return arg1 + arg2;
+}
+
+template<> constexpr int add(int arg1, int arg2) {
+  return arg1 + arg2 + 2;
+}