ClangCodeModel: Add experimental clangd support

If the user has enabled clangd (default is off), we start up one instance
per project when it is opened/changed (including build config switches),
and trigger background indexing.
So far, the index is used to provide results for locators and "Find
Usages".
Per-document functionality such as semantic highlighting and completion
is still provided by libclang.

Change-Id: I12532fca1b9c6278baab560e7238cba6189cde9f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcodemodel.qbs

@@ -11,6 +11,7 @@ QtcPlugin {
     Depends { name: "TextEditor" }
     Depends { name: "Utils" }
     Depends { name: "ClangSupport" }
+    Depends { name: "LanguageClient" }
 
     Depends { name: "libclang"; required: false }
     Depends { name: "clang_defines" }
@@ -72,6 +73,8 @@ QtcPlugin {
         "clangfollowsymbol.h",
         "clangfunctionhintmodel.cpp",
         "clangfunctionhintmodel.h",
+        "clanggloballocatorfilters.cpp",
+        "clanggloballocatorfilters.h",
         "clanghighlightingresultreporter.cpp",
         "clanghighlightingresultreporter.h",
         "clanghoverhandler.cpp",