Also bump Linux Clang baseline to 12.0.1 (was: Bump --enable-compiler-plugins Clang baseline?)

[Stephan Bergmann]

On 2/11/22 15:37, Stephan Bergmann wrote:
> If anybody has any issues with this Linux Clang-w/o-loplugin baseline 
> bump, please raise your voice now.

One thing I just clarified with Rene on IRC is that this will not affect 
any Clang only used to build external/skia (in a build otherwise using 
GCC or MSVC).  The check for that LO_CLANG_CC/LO_CLANG_CXX's version in 
configure.ac will be left unchanged at 5.0.2.