[clang-tidy] Fix extern fixes in readability-redundant-declaration

Currently check does not look into LinkageSpecDecl
when removing redundant variable re-declarations.
This were leaving code in non-compiling state.
Fix patch fixes this and adds removal also of 'extern C'.

Fixes: https://github.com/llvm/llvm-project/issues/42068

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D146904