Provide alias symbol name trait

[dlangBugzillaToGithub]

Radu Racariu (@rracariu) reported this on 2018-06-26T14:13:35Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19029

Description

Currently there is no way to get the name of the alias for a symbol type.

For example the following:
===
void main()
{
    import std.traits;
    size_t s;
    
    pragma(msg, typeof(s).stringof);
    pragma(msg, mangledName!(typeof(s)));
    pragma(msg, mangledName!s);
}
===
Prints
---
ulong
m
_D9onlineapp4mainFZ1sm
---

For code generation porpoises (eg. generate C wrapper code) there is no way to get the type of `s` as `size_t` because `size_t` is an alias.

A new trait, `__traits(aliasName, symbol)`, could be used to retrieve the alias name for a symbol type.

This trait will return the name of the concrete type if the symbol type is not an alias.