[clangd] Fix build bot after r342961
[clangd] Fix build bot after r342961

Use llvm::isAlpha instead of std::isalpha etc. This should fix bot failure:
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/20180

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@342964 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