[clang][flang][driver] Correct program names in option group descriptions (#81726)

Currently https://flang.llvm.org/docs/FlangCommandLineReference.html
refers to "Clang" in several of the group descriptions for example:
```
Compilation options

Flags controlling the behavior of Clang during compilation...
```

This is pretty confusing. I'm fixing this by making use of `Program`
from the existing GlobalDocumentation object to substitute in the
program name to these descriptions.

This `Program` has been changed to a proper noun given that it's easier
to lower case a string than capitalise one character (syntax wise). The
tablegen backend has been changed to lower it so that links in the
RST/HTML remain the same as they were before.

To make sure the file is valid when not generating docs, I'm checking a
#define and providing a default GlobalDocumentation if it's not defined.
(I looked for a way to check if a def exists, but tablegen doesn't seem
to have one)

This means that if the Do...