Re-land [clangd] Add support for missing includes analysis.

[VitaNuo]

Summary

This reverts commit 2eb5ac99a76dbbf8ac68c538211fabeaa5ac0bfd.

clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp

TEST(IncludeCleaner, ComputeMissingHeaders) {
   MissingIncludeDiagInfo BInfo{B, BRange, {Header}};
   EXPECT_THAT(Findings.MissingIncludes, ElementsAre(BInfo));
 }
 
 TEST(IncludeCleaner, GenerateMissingHeaderDiags) {
   Config Cfg;
   Cfg.Diagnostics.MissingIncludes = Config::IncludesPolicy::Strict;
   Cfg.Diagnostics.Includes.IgnoreHeader = {[](llvm::StringRef Header) {
-    return Header == testPath("buzz.h", llvm::sys::path::Style::posix);
+    return Header.ends_with("buzz.h");
   }};
   WithContextValue Ctx(Config::Key, std::move(Cfg));
   Annotations MainFile(R"cpp(
 #include "a.h"
 $insert_b[[]]#include "baz.h"
 #include "dir/c.h"
 $insert_d[[]]#include "fuzz.h"
 #include "header.h"