Remove ASan from the default build mode (#7003)

Also increases the default optimization to `-Og` which is likely to give
faster turn-around time which is what we want to optimize for here. This
should also _substantially_ shrink binary sizes, etc.

ASan is still available via `--config=asan`, and is added to the CI
infrastructure. However, my current thought is to only run it after push
rather than in PRs and in the merge queue.

docs/project/contribution_tools.md

@@ -24,6 +24,7 @@ contributions.
     -   [Optional tools](#optional-tools)
         -   [Jujutsu (`jj`)](#jujutsu-jj)
         -   [AI assistants](#ai-assistants)
+    -   [Running tests with AddressSanitizer (ASan)](#running-tests-with-addresssanitizer-asan)
     -   [Manually building Clang and LLVM (not recommended)](#manually-building-clang-and-llvm-not-recommended)
 -   [Troubleshooting build issues](#troubleshooting-build-issues)
     -   [`bazel clean`](#bazel-clean)
@@ -310,6 +311,20 @@ git show
 git status
 ```
 
+### Running tests with AddressSanitizer (ASan)
+
+By default, the Bazel build mode for the toolchain does not enable
+AddressSanitizer (ASan). If you wish to enable ASan for local testing, you must
+pass the `--config=asan` flag explicitly:
+
+```shell
+bazelisk test --config=asan //...
+```
+
+Note that our Continuous Integration (CI) infrastructure runs a separate
+configuration for ASan to ensure test coverage without slowing down the default
+test cycle.
+
 ### Manually building Clang and LLVM (not recommended)
 
 We primarily test against [apt.llvm.org](https://apt.llvm.org) and Homebrew