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

[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
error: Semantic errors in file.f90
./file.f90:2:18: error: Must be a constant value
    integer :: i = k
$ flang-new -fc1 file.f90
error: Semantic errors in file.f90
./file.f90:2:18: error: Must be a constant value
    integer :: i = k
```

Before this change, none of the above would be formatted. Note also that the
default behaviour in `flang-new` is different to `flang-new -fc1` (this is
consistent with Clang).

**NOTES ON IMPLEMENTATION**
Note that the diagnostic options are parsed in `createAndPopulateDiagOpt`s in
driver.cpp. That's where the driver's `DiagnosticEngine` options are set. Like
most command-line compiler driver options, these flags are "claimed" in
Flang.cpp (i.e.  when creating a frontend driver invocation) by calling
`getLastArg` rather than in driver.cpp.

In Clang's Options.td, `defm color_diagnostics` is replaced with two separate
definitions: `def fcolor_diagnostics` and def fno_color_diagnostics`. That's
because originally `color_diagnostics` derived from `OptInCC1FFlag`, which is a
multiclass for opt-in options in CC1. In order to preserve the current
behaviour in `clang -cc1` (i.e. to keep `-fno-color-diagnostics` unavailable in
`clang -cc1`) and to implement similar behaviour in `flang-new -fc1`, we can't
re-use `OptInCC1FFlag`.

Formatting is only available in consoles that support it and will normally mean that
the message is printed in bold + color.

Co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>
Reviewed By: rovka

Differential Revision: https://reviews.llvm.org/D126164