[clang][driver] Restore the original help text for `-I`

Authored by awarzynski on Jan 6 2021, 4:49 AM.


[clang][driver] Restore the original help text for -I

The help text for -I was recently expanded in [1]. The expanded
version focuses on explaining the semantics of -I in Clang. We are now
in the process of adding support for -I in Flang and this new
description is incompatible with the semantics of -I in Flang. This
was brought up in this review:

This patch reverts the original change in Options.td. This way the help
text for -I remains generic enough so that it applies to both Clang
and Flang.

The expanded description of -I from [1] is moved to the
DocBrief field for -I. This field is prioritised over the help text
when generating ClangCommandLineReference.rst, so the user facing
documentation for Clang retains the expanded description:

DocBrief fields are currently not used in Flang.

As requested in the reviews, the help text and the expanded description
are slightly refined.

[1] Commit: 8dd4e3ceb804a58bcf25e6856fc6fde5e1995a66

Differential Revision: https://reviews.llvm.org/D94169