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

Summary:
.clangd/index was well-intentioned in 2754942cbaef, but `.clangd` is the best
filename for the clangd config file (matching .clang-format and .clang-tidy).
And of course we can't have both .clangd/index and .clangd...

There are a few overlapping goals to satisfy:
 - it should be clear from the directory name that this is transient
   data that is safe to delete at the cost of recomputation, i.e. a cache
 - it should be easy and self-documenting to blacklist these files in .gitignore
 - we should have some consistency between filenames in-tree and
   corresponding files in user storage (e.g. under XDG's ~/.cache/)
 - we should be consistent across platforms (including windows, which
   doesn't have distinct cache vs config directories)

So the plan is:
  $PROJECT/.clangd                    (project config)
  $PROJECT/.cache/clangd/index/       (project index)
  $PROJECT/.cache/clangd/modules/     (maybe in future)
  $XDG_CONFIG_HOME/clangd/config.yaml (user config)
  $XDG_CACHE_HOME/clangd/index/       (index of non-project files)
  $XDG_CACHE_HOME/clangd/modules/     (maybe in future)
This is sensible if XDG_{CONFIG,CACHE}_HOME coincide, and has a simple
.gitignore rule going forward: `.cache/`.

The monorepo gitignore is updated to reflect the backwards-compatible practice:
  ignore .clangd/ (with trailing slash) matching index files from clangd 9/10
  ignore .cache matching index from clangd 11+, and potentially other tools.
The entries from llvm-project/llvm gitignore are removed (obsolete).

Reviewers: kadircet, hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, omtcyfz, arphaman, usaxena95, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D83099

.gitignore

@@ -53,10 +53,11 @@ autoconf/autom4te.cache
 # VS2017 and VSCode config files.
 .vscode
 .vs
-# clangd index
-.clangd
+# clangd index. (".clangd" is a config file now, thus trailing slash)
+.clangd/
+.cache
 # static analyzer regression testing project files
 /clang/utils/analyzer/projects/*/CachedSource
 /clang/utils/analyzer/projects/*/PatchedSource
 /clang/utils/analyzer/projects/*/ScanBuildResults
-/clang/utils/analyzer/projects/*/RefScanBuildResults
+/clang/utils/analyzer/projects/*/RefScanBuildResults

clang-tools-extra/clangd/index/Background.h

@@ -56,9 +56,9 @@ public:
   using Factory = llvm::unique_function<BackgroundIndexStorage *(PathRef)>;
 
   // Creates an Index Storage that saves shards into disk. Index storage uses
-  // CDBDirectory + ".clangd/index/" as the folder to save shards. CDBDirectory
-  // is the first directory containing a CDB in parent directories of a file, or
-  // user's home directory if none was found, e.g. standard library headers.
+  // CDBDirectory + ".cache/clangd/index/" as the folder to save shards.
+  // CDBDirectory is the first directory containing a CDB in parent directories
+  // of a file, or user cache directory if none was found, e.g. stdlib headers.
   static Factory createDiskBackedStorageFactory(
       std::function<llvm::Optional<ProjectInfo>(PathRef)> GetProjectInfo);
 };

clang-tools-extra/clangd/index/BackgroundIndexStorage.cpp

@@ -95,8 +95,8 @@ public:
 };
 
 // Creates and owns IndexStorages for multiple CDBs.
-// When a CDB root is found, shards are stored in $ROOT/.clangd/index.
-// When no root is found, the fallback path is ~/.cache/clangd/index.
+// When a CDB root is found, shards are stored in $ROOT/.cache/clangd/index/.
+// When no root is found, the fallback path is ~/.cache/clangd/index/.
 class DiskBackedIndexStorageManager {
 public:
   DiskBackedIndexStorageManager(
@@ -115,7 +115,7 @@ public:
     llvm::SmallString<128> StorageDir(FallbackDir);
     if (auto PI = GetProjectInfo(File)) {
       StorageDir = PI->SourceRoot;
-      llvm::sys::path::append(StorageDir, ".clangd", "index");
+      llvm::sys::path::append(StorageDir, ".cache", "clangd", "index");
     }
     auto &IndexStorage = IndexStorageMap[StorageDir];
     if (!IndexStorage)

clang-tools-extra/clangd/test/background-index.test

@@ -15,8 +15,8 @@
 # RUN: clangd -background-index -lit-test < %t/definition.jsonrpc | FileCheck %t/definition.jsonrpc --check-prefixes=CHECK,BUILD
 
 # Test that the index is writing files in the expected location.
-# RUN: ls %t/.clangd/index/foo.cpp.*.idx
-# RUN: ls %t/sub_dir/.clangd/index/foo.h.*.idx
+# RUN: ls %t/.cache/clangd/index/foo.cpp.*.idx
+# RUN: ls %t/sub_dir/.cache/clangd/index/foo.h.*.idx
 
 # Test the index is read from disk: delete code and restart clangd.
 # RUN: rm %t/foo.cpp

llvm/.gitignore

@@ -59,8 +59,6 @@ docs/_build
 # VS2017 and VSCode config files.
 .vscode
 .vs
-# clangd index
-.clangd
 
 #==============================================================================#
 # Files created in tree by the Go bindings.