[clang-doc] Fix casting not working in gcc 5.4.0

An implicit cast of std::string to llvm::SmallString<> was breaking GCC 5.4.0 builder.
A pair using llvm::SmallString<> now uses std::string.

Differential Revision: https://reviews.llvm.org/D66378

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@369182 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed