Provide signature while typing template arguments: Foo< ^here >
Here the parameters are e.g. "typename x", and the result type is e.g.
"struct" (class template) or "int" (variable template) or "bool (std::string)"
(function template).
Multiple overloads are possible when a template name is used for several
overloaded function templates.
Fixes https://github.com/clangd/clangd/issues/299
Differential Revision: https://reviews.llvm.org/D116352
Sam McCall