[clangd] Fix check-clangd with no clang built

[sammccall]

Summary

• pass required=False to use_clang(), as we don't need it
• fix required=False (which was unused and rotted):
  • make derived substitutions conditional on it
  • add a feature so we can disable tests that need it
• conditionally disable our one test that depends on %resource_dir. This doesn't seem right from first principles, but isn't a big deal.

[kbobyrev]

LGTM

[smeenai]

Thanks!

clang-tools-extra/clangd/test/document-link.test

+# for %resource_dir: REQUIRES: clang
 # %resource_dir actually points at builtin_include_dir, go up one directory.
 # RUN: clangd -lit-test -resource-dir=%resource_dir/.. < %s | FileCheck -strict-whitespace %s
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
 ---
 {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"#include <stdint.h>\n#include <stddef.h>"}}}
 ---
 {"jsonrpc":"2.0","id":2,"method":"textDocument/documentLink","params":{"textDocument":{"uri":"test:///main.cpp"}}}
 #      CHECK:  "id": 2,

clang-tools-extra/clangd/test/lit.cfg.py

 import lit.llvm
 
 lit.llvm.initialize(lit_config, config)
-lit.llvm.llvm_config.use_clang()
+lit.llvm.llvm_config.use_clang([], [], required=False)
 lit.llvm.llvm_config.use_default_substitutions()
 
 config.name = 'Clangd'
 config.suffixes = ['.test']
 config.excludes = ['Inputs']
 config.test_format = lit.formats.ShTest(not lit.llvm.llvm_config.use_lit_shell)
 config.test_source_root = config.clangd_source_dir + "/test"
 config.test_exec_root = config.clangd_binary_dir + "/test"

llvm/utils/lit/lit/llvm/config.py

def use_clang(self, additional_tool_dirs=[], additional_flags=[], required=True):
         self.with_environment('PATH', paths, append_path=True)
 
         lib_dir_props = [self.config.name.lower() + '_libs_dir', 'clang_libs_dir', 'llvm_shlib_dir', 'llvm_libs_dir']
         paths = [getattr(self.config, pp) for pp in lib_dir_props
                  if getattr(self.config, pp, None)]
 
         self.with_environment('LD_LIBRARY_PATH', paths, append_path=True)
 
-        # Discover the 'clang' and 'clangcc' to use.
-
-        self.config.clang = self.use_llvm_tool(
-            'clang', search_env='CLANG', required=required)
-
         shl = getattr(self.config, 'llvm_shlib_dir', None)
         pext = getattr(self.config, 'llvm_plugin_ext', None)
         if shl:
             self.config.substitutions.append(('%llvmshlibdir', shl))
         if pext:
             self.config.substitutions.append(('%pluginext', pext))
 
+        # Discover the 'clang' and 'clangcc' to use.
+        self.config.clang = self.use_llvm_tool(
+            'clang', search_env='CLANG', required=required)
+        if self.config.clang:
+          self.config.available_features.add('clang')
           builtin_include_dir = self.get_clang_builtin_include_dir(self.config.clang)
           tool_substitutions = [
               ToolSubst('%clang', command=self.config.clang, extra_args=additional_flags),
               ToolSubst('%clang_analyze_cc1', command='%clang_cc1', extra_args=['-analyze', '%analyze', '-setup-static-analyzer']+additional_flags),
               ToolSubst('%clang_cc1', command=self.config.clang, extra_args=['-cc1', '-internal-isystem', builtin_include_dir, '-nostdsysteminc']+additional_flags),
               ToolSubst('%clang_cpp', command=self.config.clang, extra_args=['--driver-mode=cpp']+additional_flags),
               ToolSubst('%clang_cl', command=self.config.clang, extra_args=['--driver-mode=cl']+additional_flags),
               ToolSubst('%clangxx', command=self.config.clang, extra_args=['--driver-mode=g++']+additional_flags),
               ]
           self.add_tool_substitutions(tool_substitutions)
+          self.config.substitutions.append(
+              ('%resource_dir', builtin_include_dir))
 
         self.config.substitutions.append(('%itanium_abi_triple',
                                           self.make_itanium_abi_triple(self.config.target_triple)))
         self.config.substitutions.append(('%ms_abi_triple',
                                           self.make_msabi_triple(self.config.target_triple)))
-        self.config.substitutions.append(
-            ('%resource_dir', builtin_include_dir))
 
         # The host triple might not be set, at least if we're compiling clang from
         # an already installed llvm.
         if self.config.host_triple and self.config.host_triple != '@LLVM_HOST_TRIPLE@':
             self.config.substitutions.append(('%target_itanium_abi_host_triple',
                                               '--target=%s' % self.make_itanium_abi_triple(self.config.host_triple)))
         else:
             self.config.substitutions.append(