[ycm] Fix YouCompleteMe config

The .ycm_extra_conf.py is out of date with the current build settings
and ycm versions. In particular, ycm expects a `Settings` function,
a couple of more flags need to be passed and some includes are marked
-isystem now. Also, we can use the C++ standard from the ninja files
instead of hard-coding it.

Change-Id: Ibaed29eb341346a86b6b736f95c97b41957f62de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3980257
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Olivier Flückiger <olivf@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#84013}

.ycm_extra_conf.py

@@ -42,7 +42,6 @@ import sys
 # Flags from YCM's default config.
 flags = [
 '-DUSE_CLANG_COMPLETER',
-'-std=gnu++14',
 '-x',
 'c++',
 ]
@@ -143,25 +142,27 @@ def GetClangCommandFromNinjaForFilename(v8_root, filename):
   # Parse flags that are important for YCM's purposes.
   for flag in clang_line.split(' '):
     if flag.startswith('-I'):
-      # Relative paths need to be resolved, because they're relative to the
-      # output dir, not the source.
-      if flag[2] == '/':
-        v8_flags.append(flag)
-      else:
-        abs_path = os.path.normpath(os.path.join(out_dir, flag[2:]))
-        v8_flags.append('-I' + abs_path)
-    elif flag.startswith('-std'):
+      v8_flags.append(MakeIncludePathAbsolute(flag, "-I", out_dir))
+    elif flag.startswith('-isystem'):
+      v8_flags.append(MakeIncludePathAbsolute(flag, "-isystem", out_dir))
+    elif flag.startswith('-std') or flag.startswith(
+        '-pthread') or flag.startswith('-no'):
       v8_flags.append(flag)
-    elif flag.startswith('-') and flag[1] in 'DWFfmO':
-      if flag == '-Wno-deprecated-register' or flag == '-Wno-header-guard':
-        # These flags causes libclang (3.3) to crash. Remove it until things
-        # are fixed.
-        continue
+    elif flag.startswith('-') and flag[1] in 'DWFfmgOX':
       v8_flags.append(flag)
-
   return v8_flags
 
 
+def MakeIncludePathAbsolute(flag, prefix, out_dir):
+  # Relative paths need to be resolved, because they're relative to the
+  # output dir, not the source.
+  if flag[len(prefix)] == '/':
+    return flag
+  else:
+    abs_path = os.path.normpath(os.path.join(out_dir, flag[len(prefix):]))
+    return prefix + abs_path
+
+
 def FlagsForFile(filename):
   """This is the main entry point for YCM. Its interface is fixed.
 
@@ -180,3 +181,9 @@ def FlagsForFile(filename):
     'flags': final_flags,
     'do_cache': True
   }
+
+
+def Settings(**kwargs):
+  if kwargs['language'] == 'cfamily':
+    return FlagsForFile(kwargs['filename'])
+  return {}