Capture macro expansions in an output group named macro_expansions …

…during debug builds

LLDB wants these files to be present on the file system to step into macro expansions, because their file paths are recorded in debug info. The compiler uses `TMPDIR` (if set) as the location for those files, so we can have the worker set `TMPDIR` to a target-specific location before spawning `swiftc`. If this location is workspace-relative, then those file paths are also remapped via `-debug-prefix-map`—an added bonus.

The `macro_expansions` group will contain a tree (directory) artifact that contains a `swift-generated-sources` subdirectory (the compiler forces that name), which contains one or more `.swift` files representing the expanded macro buffers.

PiperOrigin-RevId: 549932429
(cherry picked from commit 69349ee)

Cherry-pick notes: We already included most of this in c9d1f5f. This just restricts `_macro_expansion_configurator` to debug builds.

Signed-off-by: Brentley Jones <github@brentleyjones.com>

swift/toolchains/config/compile_config.bzl

@@ -811,6 +811,10 @@ def compile_action_configs(
                 SWIFT_ACTION_DERIVE_FILES,
             ],
             configurators = [_macro_expansion_configurator],
+            # The compiler only generates these in debug builds, unless we pass
+            # additional frontend flags. At the current time, we only want to
+            # capture these for debug builds.
+            not_features = [SWIFT_FEATURE_OPT],
         ),
 
         # swift-symbolgraph-extract doesn't yet support explicit Swift module