remove -fpch-codegen and -fpch-debuginfo from Clang 12.0 release notes

These were new in 11.0. The commit adding the options landed after
11.x branch had already been branched off from master, and only
then backported to 11.x, so the release notes change stayed for 12.0.

clang/docs/ReleaseNotes.rst

@@ -82,31 +82,6 @@ New Compiler Flags
   in that case. The option's behaviour mirrors GCC, the helpers are implemented
   both in compiler-rt and libgcc.
 
-- -fpch-codegen and -fpch-debuginfo generate shared code and/or debuginfo
-  for contents of a precompiled header in a separate object file. This object
-  file needs to be linked in, but its contents do not need to be generated
-  for other objects using the precompiled header. This should usually save
-  compile time. If not using clang-cl, the separate object file needs to
-  be created explicitly from the precompiled header.
-  Example of use:
-
-  .. code-block:: console
-
-    $ clang++ -x c++-header header.h -o header.pch -fpch-codegen -fpch-debuginfo
-    $ clang++ -c header.pch -o shared.o
-    $ clang++ -c source.cpp -o source.o -include-pch header.pch
-    $ clang++ -o binary source.o shared.o
-
-  - Using -fpch-instantiate-templates when generating the precompiled header
-    usually increases the amount of code/debuginfo that can be shared.
-  - In some cases, especially when building with optimizations enabled, using
-    -fpch-codegen may generate so much code in the shared object that compiling
-    it may be a net loss in build time.
-  - Since headers may bring in private symbols of other libraries, it may be
-    sometimes necessary to discard unused symbols (such as by adding
-    -Wl,--gc-sections on ELF platforms to the linking command, and possibly
-    adding -fdata-sections -ffunction-sections to the command generating
-    the shared object).
 - New option ``-fbinutils-version=`` specifies the targeted binutils version.
   For example, ``-fbinutils-version=2.35`` means compatibility with GNU as/ld
   before 2.35 is not needed: new features can be used and there is no need to