Make it easier to read/write the template part of FunctionDecl.

Introduce:
-FunctionDecl::getTemplatedKind() which returns an enum signifying what kind of templated
  FunctionDecl it is.
-An overload of FunctionDecl::setFunctionTemplateSpecialization() which accepts arrays of
  TemplateArguments and TemplateArgumentLocs
-A constructor to TemplateArgumentList which accepts an array of TemplateArguments.

llvm-svn: 106532