[CODEOWNERS] Add code owners for PropertySetIO and Base64 (#3098)

Assign code author (@kbobrovs) and primary owner and Alexey Sachkov as a
back-up.

Author: bader

.github/CODEOWNERS

@@ -99,6 +99,10 @@ llvm/**/file-table-tform/ @kbobrovs @AlexeySachkov
 llvm/**/llvm-foreach/ @AlexeySachkov  @Fznamznon
 llvm/**/llvm-no-spir-kernel/ @AGindinson @AlexeySachkov
 llvm/**/sycl-post-link/ @kbobrovs @AlexeySachkov
+llvm/include/llvm/Support/PropertySetIO.h @kbobrovs @AlexeySachkov
+llvm/lib/Support/PropertySetIO.cpp @kbobrovs @AlexeySachkov
+llvm/unittests/Support/PropertySetIOTest.cpp @kbobrovs @AlexeySachkov
+llvm/lib/Support/Base64.cpp @kbobrovs @AlexeySachkov
 
 # Clang offload tools
 clang/tools/clang-offload-bundler/ @kbobrovs @sndmitriev