Always build a new TypeSourceInfo for function templates with parameters

RecursiveASTVisitor::TraverseFunctionHelper() traverses a function's
ParmVarDecls by going to the function's getTypeSourceInfo if it exists, and
`DEF_TRAVERSE_TYPELOC(FunctionProtoType` then goes to the function's
ParmVarDecls.

For a function template that doesn't have parameters that explicitly depend on
the template parameter, we used to be clever and not build a new
TypeSourceInfo. That meant that when an instantiation of such a template is
visited, its TypeSourceInfo would point to the ParmVarDecls of the template,
not of the instantiation, which then confused clients of RecursiveASTVisitor.

So don't be clever for function templates that have parameters, even if none of
the parameters depend on the type.

Fixes PR26257.
http://reviews.llvm.org/D16478

llvm-svn: 259428