Gets the full signature of this function, including return type, parameter
types, and template arguments.
For example, in the following code:
template<typename T> T min(T x, T y); int z = min(5, 7);
The full signature of the function called on the last line would be
min<int>(int, int) -> int, and the full signature of the uninstantiated
template on the first line would be
min<T>(T, T) -> T.