[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