[clangd-dev] Tooling RFC: Find build root from source root with a symlink?

[Sam McCall via clangd-dev]

Many projects/build systems have the concept of a "build tree" that can be
separate from the source tree. (e.g. CMake supports this, and LLVM
recommends this approach).

Tools sometimes need to find the build tree.
I'd like to suggest a convention for the Tooling library to support:
    If $SRC/buildroot exists, it's the default build directory for the
source tree $SRC.
    Otherwise, the build directory is $SRC itself.
    $SRC/buildroot may be a symlink.
    Configuration files like compile_commands.json are searched for in the
build directory.
(Bikeshedding the "buildroot" string is welcome. "build" will conflict too
often, sadly).

Implications:
 - for /foo/bar/baz.cc, we'd search for compile_commands.json in
{/foo/bar/buildroot/,/foo/bar/, /foo/buildroot/, etc}. This is
backwards-compatible.
 - where users currently symlink compile_commands.json itself, they could
create the buildroot symlink instead. This would work in the same way, and
enable new cases.
 - if we implement e.g. a Ninja-backed compilation DB that doesn't need
compile_commands.json, the same symlink convention would work.
 - it provides a simple model for multi-configuration-aware tools: a
configuration is defined by a build dir, there's a default configuration,
tools can let the user override the build dir. e.g. clangd can write its
index files into the build dir instead of the source dir, which is
multi-configuration-friendly (tinyurl.com/clangd-automatic-index)
 - non-clang tools that consume compile_commands.json will need to be
updated over time.

What do you think?
Cheers, Sam
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