Add new option -fkeep-persistent-storage-variables to Clang release notes

[qianzhen]

Summary

This patch updates the Clang release notes with the new option -fkeep-persistent-storage-variables added at https://reviews.llvm.org/D150221.

[hubert.reinterpretcast]

clang/docs/ReleaseNotes.rst

278–279

Suggestion: Use em dashes to avoid overuse of commas.

Additionally, I am wondering if the release notes is a reasonable place to add something like: Since this inhibits the merging of the affected variables, the number of individual relocations in the program will generally increase.

[qianzhen]

Update description

[hubert.reinterpretcast]

LGTM; thanks!

clang/docs/ReleaseNotes.rst

 ------------------
 - The flag ``-std=c++23`` has been added. This behaves the same as the existing
   flag ``-std=c++2b``.
 - ``-dumpdir`` has been implemented to specify auxiliary and dump output
   filenames for features like ``-gsplit-dwarf``.
 - ``-fcaret-diagnostics-max-lines=`` has been added as a driver options, which
   lets users control the maximum number of source lines printed for a
   caret diagnostic.
+- ``-fkeep-persistent-storage-variables`` has been implemented to keep all
+  variables that have a persistent storage duration—including global, static
+  and thread-local variables—to guarantee that they can be directly addressed.
+  Since this inhibits the merging of the affected variables, the number of
+  individual relocations in the program will generally increase.
 
 Deprecated Compiler Flags
 -------------------------
 
 - ``-fdouble-square-bracket-attributes`` has been deprecated. It is ignored now
   and will be removed in Clang 18.
 
 Modified Compiler Flags