[Dlang-internal] Separate compilation identifier mess and the bug trail that ensued

[kinke]

On Wednesday, 4 July 2018 at 22:15:36 UTC, Atila Neves wrote:
> Using `static foreach` or mixing in unittests with strings 
> meant that the different unittests shared line and column 
> numbers. Oops.

To make those corner cases unique and independent from compiler 
invokations, maybe a stack of module-line-column infos could be 
used instead of a single one. During semantic, each mixin would 
push its Loc, and each static foreach iteration would push an 
artificial one incl. iteration index.