clang tooling: Make script arguments more consistent and update docs

- all: Use -p consistently to specify a path to the compile db to match
  the convention of clang::tooling::ClangTool
- all: Default -p to out/Debug for convenience.
- docs: Update docs to reference renamed arguments
- docs: Update clang-tidy documentation to use the compile DB generation
  wrapper script
- run_tool.py: Change tool to a non-positional argument and make it a
  required argument.

Change-Id: I378402cab36e86da4b13fb43e26067cad4db1795
Reviewed-on: https://chromium-review.googlesource.com/494946
Commit-Queue: Daniel Cheng <[email protected]>
Reviewed-by: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/master@{#469409}
diff --git a/docs/clang_tidy.md b/docs/clang_tidy.md
index 756f609..385d26e 100644
--- a/docs/clang_tidy.md
+++ b/docs/clang_tidy.md
@@ -53,7 +53,7 @@
 ```
 2.  Generate the compilation database
 ```
-ninja -C out/Release -t compdb objcxx cxx > compile_commands.json
+tools/clang/scripts/generate_compdb.py -p out/Release > compile_commands.json
 ```
 3.  Enter the build directory.
 ```