[-cxx-abi microsoft] Properly mangle enums

While testing our ability to mangle large constants (PR18175), I
incidentally discovered that we did not properly mangle enums correctly.

Previously, we would append the width of the enum in bytes after the
type-tag differentiator.

This would mean "enum : short" would be mangled as 'W2' while "enum :
char" would be mangled as 'W1'.  Upon testing this with several versions
of MSVC, I found that this did not match their behavior: they always use
'W4'.

N.B.  Quick testing uncovered that undname allows different numbers to
follow the 'W' in the following way:

'W0' -> "enum char"
'W1' -> "enum unsigned char"
'W2' -> "enum short"
'W3' -> "enum unsigned short"
'W4' -> "enum"
'W5' -> "enum unsigned int"
'W6' -> "enum long"
'W7' -> "enum unsigned long"

However this scheme appears abandoned, I cannot get MSVC to trigger it.
Furthermore, it's incomplete: it doesn't handle "bool" or "long long".