Require C++14 when building compilerplugins

[Stephan Bergmann]

For those building LO's Clang compilerplugins:  LLVM/Clang trunk has 
switched to C++14 for building, and that makes it harder to maintain a 
compilerplugins codebase that can be built both against old versions of 
LLVM/Clang, which only required -std=c++11 in COMPILER_PLUGINS_CXX, and 
against LLVM/Clang trunk.

Hence, <https://gerrit.libreoffice.org/#/c/77878/> "Require C++14 when 
building compilerplugins" proposes that we unconditionally make 
-std=c++14 the minimum in COMPILER_PLUGINS_CXX for building 
compilerplugins on master.

Any objections?