[clang-tools-extra] [clangd] Add new highlighting modifier `CommandLineDefined`. (PR #175495)

[via cfe-commits]

anonymouspc wrote:

> Can you elaborate on the motivation for this? What are some scenarios in which it's necessary or important for a user to distinguish between macros defined in source code and macros defined on the command-line?

- **header-defined** macros, such as
  - `stdin`, `offsetof`
  - `UINT8_MAX`
  - `BOOST_FUSION_ADAPT_STRUCT`, `BOOST_PP_REPEAT`
  - `__cpp_concepts`, `__cpp_lib_ranges`.
 
  They are **always defined** in different build type.
  Editor/IDEs can always render `#ifdef __cpp_concepts` gray when `-std=c++17`.

- **command-line-defined** macros, such as
  - `DEBUG`, `NDEBUG`
  - `PKG_STATIC`, `PKG_SHARED`,
  
  They are often **defined in certain build type**.
  Even if `NDEBUG` is not defined, Editor/IDEs should not render the code between `#ifndef NDEBUG` gray.

https://github.com/llvm/llvm-project/pull/175495