[clangd] Fix a bug in asynchronous code completion
[clangd] Fix a bug in asynchronous code completion

A StringRef that goes out of scope was copied and used in a handler
running on a separate thread.
We didn't catch it because clangd does not use the async completion
API yet.

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@322080 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