[clang-doc] Fix warnings
This patch fixes:
clang-tools-extra/clang-doc/BitcodeReader.cpp:47:20: error: unused
function 'decodeRecord' [-Werror,-Wunused-function]
clang-tools-extra/clang-doc/BitcodeReader.cpp:437:31: error: unused
function 'getCommentInfo<std::unique_ptr<clang::doc::CommentInfo>
&>' [-Werror,-Wunused-function]
