Function call mangling again

[Jason Merrill]

While looking at some late return type manglings, I noticed another 
issue with function calls: now we know how to mangle the arguments, but 
we still need to specify how we mangle what's being called.  For an 
expression that contains a call to a template function "f", g++ will 
currently emit L_Z1fE, trying to use the mangled name of the template. 
I think just using source-name for a dependent name, like "1f", would be 
more appropriate, though I haven't looked to see if that creates 
ambiguities yet.

Jason