[clang] docs(clang): add some missing flags (PR #209519)

[Balázs Benics via cfe-commits]

================
@@ -926,6 +948,32 @@ information can be included in the remarks (see
 These are options that report execution time and consumed memory of different
 compilations steps.
 
+```{eval-rst}
+.. option:: -ftime-report-json
+
+  Print compiler pass timing statistics as a JSON object to standard error.
+  Use this when no compiler diagnostics will also be emitted to standard error, since other diagnostic output would interleave with the JSON:
+
+  .. code-block:: console
+
+    $ clang -O2 -c foo.c -ftime-report-json -o foo.o 2> timing.json
+```
----------------
steakhal wrote:

Given that you document `-ftime-trace`, should we just drop `-ftime-report-json`?
Having multiple overlapping options might be confusing to the user. I wonder what's your opinion?

https://github.com/llvm/llvm-project/pull/209519