[clang-doc] Explicitly cast to unique_ptr
[clang-doc] Explicitly cast to unique_ptr

Older compilers don't like the implicit cast & move when returning a
unique_ptr to an llvm::Expected type.

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@339785 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra