[clangd] No crash on "-verify" mode.

[hokein]

Summary

If there is a "-verify" flag in the compile command, clangd will crash
(hit the assertion) inside the ~VerifyDiagnosticConsumer (Looks like our
compiler invocation doesn't setup correctly?).

This patch disables the verify mode as it is rarely useful in clangd.

clang-tools-extra/clangd/Compiler.cpp

std::unique_ptr<CompilerInvocation> CI = createInvocationFromCommandLine(
       ArgStrs, CommandLineDiagsEngine, std::move(VFS),
       /*ShouldRecoverOnErrors=*/true, CC1Args);
   if (!CI)
     return nullptr;
   // createInvocationFromCommandLine sets DisableFree.
   CI->getFrontendOpts().DisableFree = false;
   CI->getLangOpts()->CommentOpts.ParseAllComments = true;
   CI->getLangOpts()->RetainCommentsFromSystemHeaders = true;
+  // Disable "clang -verify" diagnostics, they are rarely useful in clangd, and
+  // our compiler invocation set-up doesn't seem to work with it (leading
+  // assertions in VerifyDiagnosticConsumer).
+  CI->getDiagnosticOpts().VerifyDiagnostics = false;
 
   // Disable any dependency outputting, we don't want to generate files or write
   // to stdout/stderr.
   CI->getDependencyOutputOpts().ShowIncludesDest =
       ShowIncludesDestination::None;
   CI->getDependencyOutputOpts().OutputFile.clear();
   CI->getDependencyOutputOpts().HeaderIncludeOutputFile.clear();
   CI->getDependencyOutputOpts().DOTOutputFile.clear();

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

#else
       int x = y;
     #endif
     )cpp");
   EXPECT_THAT(
       TestTU::withCode(Test.code()).build().getDiagnostics(),
       ElementsAre(Diag(Test.range(), "use of undeclared identifier 'b'")));
 }
 
+TEST(DiagnosticsTest, IgnoreVerify) {
+  auto TU = TestTU::withCode(R"cpp(
+    int a; // expected-error {{}}
+  )cpp");
+  TU.ExtraArgs.push_back("-Xclang");
+  TU.ExtraArgs.push_back("-verify");
+  EXPECT_THAT(TU.build().getDiagnostics(), IsEmpty());
+}
+
 // Recursive main-file include is diagnosed, and doesn't crash.
 TEST(DiagnosticsTest, RecursivePreamble) {
   auto TU = TestTU::withCode(R"cpp(
     #include "foo.h" // error-ok
     int symbol;
   )cpp");
   TU.Filename = "foo.h";
   EXPECT_THAT(TU.build().getDiagnostics(),