Make the 'resource-dir' flag public

Currently the "getter" flag '-print-resource-dir' is public, but the
corresponding "setter" flag '-resource-dir' is not.

This flag is useful when one is using clang on a non-standard location,
or when one is building a libtooling-based tool (e.g. IWYU) based on
a local clang build. In that case, we need to specify the correct
path to the resource directory for things to work.

Existing documentation already makes use of this flag, for example
here:

https://clang.llvm.org/docs/StandardCPlusPlusModules.html#possible-issues-failed-to-find-system-headers

There is thus no reason to keep this flag hidden.

Author: Carlos Gálvez

clang/docs/ReleaseNotes.rst

@@ -276,7 +276,7 @@ New Compiler Flags
 - New option ``-fno-sanitize-debug-trap-reasons`` added to disable emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).
 - New option ``-fsanitize-debug-trap-reasons=`` added to control emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).
 - New options for enabling allocation token instrumentation: ``-fsanitize=alloc-token``, ``-falloc-token-max=``, ``-fsanitize-alloc-token-fast-abi``, ``-fsanitize-alloc-token-extended``.
-
+- The ``-resource-dir`` option is now displayed in the list of options shown by ``--help``.
 
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