[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/test.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -std=c++1z -emit-pch -o %t.1.ast %S/Inputs/function-1.cpp
+// RUN: %clang_cc1 -std=c++1z  -ast-merge %t.1.ast -fsyntax-only %s 2>&1 | FileCheck %s
+// XFAIL: *
+
+static_assert(add(1, 2) == 5);
+
+// FIXME: support of templated function overload is still not implemented.
+static_assert(add('\1', '\2') == 3);
+
+// CHECK-NOT: static_assert