-cov LOC is inadequate for 1 liner branching; need a metric based on branching

[Timothee Cour]

just filed https://issues.dlang.org/show_bug.cgi?id=18377:

`dmd -cov -run main.d` shows 100% coverage; this is misleading since a
branch is not taken:

```
void main(){
  int a;
  if(false) a+=10;
}
```

how about adding a `-covmode=[loc|branch]` that would allow either
reporting LOC coverage or branch coverage?

branch coverage would report number of branches taken at least once /
total number of branches.

It would not only address the above issue, but it is IMO a much better
metric for coverage, less sensitive to 'overcounting' of large blocks
in main code branches (size of code block in a branch is irrelevant as
far as testing is concerned); eg:

```
int fun(int x){
  if(x<0)
    return fun2();  // accounts for 1 LOC and 1 branch
  // long block of non-branching code here... // accounts for 10 LOC
and 1 branch
}
```

NOTE: branches would include anything that allows more than 1 code
path (eg: switch, if)