[ReleaseNotes] Mention -fmacro-prefix-map and -ffile-prefix-map.

[MaskRay]

clang/docs/ReleaseNotes.rst

142

It affects __FILE__ (C99) __BASE_FILE__ (GNU extension) and __FILE_NAME__ (clang extension).

Should such change be committed directly to the release/10.x branch by @hans ?

[hans]

Should such change be committed directly to the release/10.x branch by @hans ?

Peter can commit to release/10.x himself, or let me know if he prefers me to do it. Thanks for writing release notes!

[Lekensteyn]

Updated the comment based on @MaskRay's feedback, thanks!
This (and the previous) patch were based on release/10.x, I plan to commit it later today since the final tag is tomorrow.

clang/docs/ReleaseNotes.rst

 
 - The -fno-concept-satisfaction-caching can be used to disable caching for
   satisfactions of Concepts. The C++2a draft standard does not currently permit
   this caching, but disabling it may incur significant compile-time costs. This
   flag is intended for experimentation purposes and may be removed at any time;
   please let us know if you encounter a situation where you need to specify this
   flag for correct program behavior.
 
+- ``-fmacro-prefix-map=OLD=NEW`` substitutes directory prefix ``OLD`` for
+  ``NEW`` in predefined preprocessor macros such as ``__FILE__``. This helps
+  with reproducible builds that are location independent. The new
+  ``-ffile-prefix-map`` option is equivalent to specifying both
+  ``-fdebug-prefix-map`` and ``-fmacro-prefix-map``.
+
 Deprecated Compiler Flags
 -------------------------
 
 The following options are deprecated and ignored. They will be removed in
 future versions of Clang.
 
 - -mmpx used to enable the __MPX__ preprocessor define for the Intel MPX
   instructions. There were no MPX intrinsics.