Add test case to cover version.c.
Print debug/version level instead of __anonymous in error messages.

When integer debug/version levels are used, ident is not defined,
causing Dsymbol::toChars() to print "__anonymous", which is unhelpful.
Instead, it should return the debug/version level being set as an
integer.
Merge pull request #4171 from quickfur/issue13786_version

Issue 13786: Add test case to cover version.c.