[flang][Driver] Refine _when_ driver diagnostics are formatted

This patch refines //when// driver diagnostics are formatted so that
`flang-new` and `flang-new -fc1` behave consistently with `clang` and
`clang -cc1`, respectively. This change only applies to driver diagnostics.
Scanning, parsing and semantic diagnostics are separate and not covered here.

**NEW BEHAVIOUR**
To illustrate the new behaviour, consider the following input file:
```! file.f90
program m
  integer :: i = k
end
```
In the following invocations, "error: Semantic errors in file.f90" _will be_
formatted:
```
$ flang-new file.f90
error: Semantic errors in file.f90
./file.f90:2:18: error: Must be a constant value
    integer :: i = k
$ flang-new -fc1 -fcolor-diagnostics file.f90
error: Semantic errors in file.f90
./file.f90:2:18: error: Must be a constant value
    integer :: i = k
```

However, in the following invocations, "error: Semantic errors in file.f90"
_will not be_ formatted:
```
$ flang-new -fno-color-diagnostics file.f90
err...