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/coreplugin/icore.cpp

@@ -531,6 +531,14 @@ QString ICore::clangExecutable(const QString &clangBinDirectory)
     return clangBinary("clang", clangBinDirectory);
 }
 
+/*!
+    \internal
+*/
+QString ICore::clangdExecutable(const QString &clangBinDirectory)
+{
+    return clangBinary("clangd", clangBinDirectory);
+}
+
 /*!
     \internal
 */