[clang] [Clang] Enable -fextend-lifetimes at -Og (PR #118026)

[Stephen Tozer via cfe-commits]

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@@ -412,6 +412,20 @@ New Compiler Flags
   only for thread-local variables, and none (which corresponds to the
   existing ``-fno-c++-static-destructors`` flag) skips all static
   destructors registration.
+- The ``-fextend-variable-liveness`` flag has been added to allow for improved
+  debugging of optimized code. Using ``-fextend-variable-liveness`` will cause
+  Clang to generate code that tries to preserve the liveness of source variables
+  through optimizations, meaning that variables will typically be visible in a
+  debugger more often. The flag has two levels: ``-fextend-variable-liveness``,
+  or ``-fextend-variable-liveness=all``, extendes the liveness of all user
+  variables and the ``this`` pointer. Alternatively ``-fextend-this-ptr``, or
+  ``-fextend-variable-liveness=this``, has the same behaviour but applies only
+  to the ``this`` variable in C++ class member functions, meaning its effect is
+  a strict subset of ``-fextend-variable-liveness``. Note that this flag
+  modifies the results of optimizations that Clang performs, which will result
+  in reduced performance in generated code; however, this feature will not
+  extend the liveness of some variables in cases where doing so would likely
+  have a severe impact on generated code performance.
----------------
SLTozer wrote:

The intention here is just to specify to the user that some variables won't have their liveness extended, so that they're not caught off-guard if they notice some variables getting excluded.

https://github.com/llvm/llvm-project/pull/118026