[clangd] Fix a regression issue caused by r348365.

[hokein]

Summary

With r348365, we now detect libc++ dir using the actual compiler path
(from the compilation command), rather than the resource-dir.

This new behavior will cause clangd couldn't find libc++ dir (even the libc++ is
built from the source) when using a fallback compilation command (clang xxx)

The fix is to use <clangd_install_dir>/clang as the actual compiler path.

[ilya-biryukov]

LGTM. Thanks

clangd/GlobalCompilationDatabase.cpp

 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
 using namespace llvm;
 namespace clang {
 namespace clangd {
 
+static std::string getFallbackClangPath() {
+  static int Dummy;
+  std::string ClangdExecutable =
+      llvm::sys::fs::getMainExecutable("clangd", (void *)&Dummy);
+  SmallString<128> ClangPath;
+  ClangPath = llvm::sys::path::parent_path(ClangdExecutable);
+  llvm::sys::path::append(ClangPath, "clang");
+  return ClangPath.str();
+}
+
 tooling::CompileCommand
 GlobalCompilationDatabase::getFallbackCommand(PathRef File) const {
-  std::vector<std::string> Argv = {"clang"};
+  std::vector<std::string> Argv = {getFallbackClangPath()};
   // Clang treats .h files as C by default, resulting in unhelpful diagnostics.
   // Parsing as Objective C++ is friendly to more cases.
   if (sys::path::extension(File) == ".h")
     Argv.push_back("-xobjective-c++-header");
   Argv.push_back(File);
   return tooling::CompileCommand(sys::path::parent_path(File),
                                  sys::path::filename(File), std::move(Argv),
                                  /*Output=*/"");

unittests/clangd/GlobalCompilationDatabaseTests.cpp

 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
 namespace clang {
 namespace clangd {
 namespace {
 using ::testing::ElementsAre;
+using ::testing::EndsWith;
 
 TEST(GlobalCompilationDatabaseTest, FallbackCommand) {
   DirectoryBasedGlobalCompilationDatabase DB(None);
   auto Cmd = DB.getFallbackCommand(testPath("foo/bar.cc"));
   EXPECT_EQ(Cmd.Directory, testPath("foo"));
-  EXPECT_THAT(Cmd.CommandLine, ElementsAre("clang", testPath("foo/bar.cc")));
+  EXPECT_THAT(Cmd.CommandLine, ElementsAre(
+    EndsWith("clang"), testPath("foo/bar.cc")));
   EXPECT_EQ(Cmd.Output, "");
 
   // .h files have unknown language, so they are parsed liberally as obj-c++.
   Cmd = DB.getFallbackCommand(testPath("foo/bar.h"));
-  EXPECT_THAT(Cmd.CommandLine, ElementsAre("clang", "-xobjective-c++-header",
-                                           testPath("foo/bar.h")));
+  EXPECT_THAT(Cmd.CommandLine,
+              ElementsAre(EndsWith("clang"), "-xobjective-c++-header",
 }
 
 static tooling::CompileCommand cmd(StringRef File, StringRef Arg) {
   return tooling::CompileCommand(testRoot(), File, {"clang", Arg, File}, "");
 }
 
 class OverlayCDBTest : public ::testing::Test {
   class BaseCDB : public GlobalCompilationDatabase {

 TEST_F(OverlayCDBTest, NoBase) {
   OverlayCDB CDB(nullptr, {"-DA=6"});
   EXPECT_EQ(CDB.getCompileCommand(testPath("bar.cc")), None);
   auto Override = cmd(testPath("bar.cc"), "-DA=5");
   CDB.setCompileCommand(testPath("bar.cc"), Override);
   EXPECT_EQ(CDB.getCompileCommand(testPath("bar.cc")), Override);
 
   EXPECT_THAT(CDB.getFallbackCommand(testPath("foo.cc")).CommandLine,
-              ElementsAre("clang", testPath("foo.cc"), "-DA=6"));
+              ElementsAre(EndsWith("clang"), testPath("foo.cc"), "-DA=6"));
 }
 
 TEST_F(OverlayCDBTest, Watch) {
   OverlayCDB Inner(nullptr);
   OverlayCDB Outer(&Inner);
 
   std::vector<std::vector<std::string>> Changes;
   auto Sub = Outer.watch([&](const std::vector<std::string> &ChangedFiles) {