fix: non-type template (enum) instantiation for templ func

[Vipul-Cariappa]

requires test:

import cppyy
from cppyy import gbl

cppyy.cppdef(
    r"""
enum What { NO, YES };

template <What E>
struct EE {
    What w = E;
};

template <What E>
What get() {
    return E;
}
"""
)

assert gbl.EE[gbl.What.NO]().w == 0
assert gbl.EE[gbl.What.YES]().w == 1

assert gbl.get[gbl.What.NO]() == 0
assert gbl.get[gbl.What.YES]() == 1

This was discovered while working on ROOT workshop slides.

[Vipul-Cariappa]

Added the test to compiler-research/cppyy#164

[vgvassilev]

Why do we need to do string parsing to do a simple-ish lookup.