CCLS Support added (#975)

Also added a test suite for both clangd and ccls

test/resources/repos/cpp/test_repo/a.cpp

@@ -0,0 +1,6 @@
+#include "b.hpp"
+
+int main() {
+    int x = add(3, 4);
+    return x;
+}

test/resources/repos/cpp/test_repo/b.cpp

@@ -0,0 +1,5 @@
+#include "b.hpp"
+
+int add(int a, int b) {
+    return a + b;
+}

test/resources/repos/cpp/test_repo/b.hpp

@@ -0,0 +1,3 @@
+#pragma once
+
+int add(int a, int b);

test/resources/repos/cpp/test_repo/compile_commands.json

@@ -0,0 +1,12 @@
+[
+  {
+    "directory": ".",
+    "command": "g++ -I . -c a.cpp",
+    "file": "a.cpp"
+  },
+  {
+    "directory": ".",
+    "command": "g++ -I . -c b.cpp",
+    "file": "b.cpp"
+  }
+]