[clangd] Store index in '.clangd/index' instead of '.clangd-index'

Summary: To take up the .clangd folder for other potential uses in the future.

Reviewers: kadircet, sammccall

Reviewed By: kadircet

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58440

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354505 91177308-0d34-0410-b5e6-96231b3b80d8

Author: ilya-biryukov

.gitignore

@@ -72,8 +72,8 @@ docs/_build
 # VS2017 and VSCode config files.
 .vscode
 .vs
-# clangd background index
-.clangd-index
+# clangd index
+.clangd
 
 #==============================================================================#
 # Files created in tree by the Go bindings.