Factor out -index-ignore-system-modules into a separate feature

The feature has opposite polarity to the compiler flag, as we want it to be off by default and only enabled on certain targets.

PiperOrigin-RevId: 467693984
(cherry picked from commit 14e066f)

---

Cherry-pick notes: We already had this flag, just named the opposite way. Not worth the breaking change to rename it. Though we are now enabling the flag by default.

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

swift/internal/feature_names.bzl

@@ -73,7 +73,9 @@ SWIFT_FEATURE_EMIT_C_MODULE = "swift.emit_c_module"
 # https://docs.google.com/document/d/1cH2sTpgSnJZCkZtJl1aY-rzy4uGPcrI-6RrUpdATO2Q/
 SWIFT_FEATURE_INDEX_WHILE_BUILDING = "swift.index_while_building"
 
-# If enabled the compilation action will not produce indexes for system modules.
+# If enabled alongside `swift.index_while_building`, the indexstore will not
+# contain records for symbols in system modules imported by the code being
+# indexed.
 SWIFT_FEATURE_DISABLE_SYSTEM_INDEX = "swift.disable_system_index"
 
 # Index while building - using a global index store cache

swift/toolchains/swift_toolchain.bzl

@@ -51,6 +51,7 @@ load(
     "SWIFT_FEATURE_CACHEABLE_SWIFTMODULES",
     "SWIFT_FEATURE_COVERAGE_PREFIX_MAP",
     "SWIFT_FEATURE_DEBUG_PREFIX_MAP",
+    "SWIFT_FEATURE_DISABLE_SYSTEM_INDEX",
     "SWIFT_FEATURE_EMIT_SWIFTDOC",
     "SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
@@ -462,6 +463,7 @@ def _swift_toolchain_impl(ctx):
         SWIFT_FEATURE_CACHEABLE_SWIFTMODULES,
         SWIFT_FEATURE_COVERAGE_PREFIX_MAP,
         SWIFT_FEATURE_DEBUG_PREFIX_MAP,
+        SWIFT_FEATURE_DISABLE_SYSTEM_INDEX,
         SWIFT_FEATURE_EMIT_SWIFTDOC,
         SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO,
         SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,

swift/toolchains/xcode_swift_toolchain.bzl

@@ -53,6 +53,7 @@ load(
     "SWIFT_FEATURE_COVERAGE_PREFIX_MAP",
     "SWIFT_FEATURE_DEBUG_PREFIX_MAP",
     "SWIFT_FEATURE_DISABLE_SWIFT_SANDBOX",
+    "SWIFT_FEATURE_DISABLE_SYSTEM_INDEX",
     "SWIFT_FEATURE_EMIT_SWIFTDOC",
     "SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO",
     "SWIFT_FEATURE_ENABLE_BATCH_MODE",
@@ -670,6 +671,7 @@ def _xcode_swift_toolchain_impl(ctx):
         SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO,
         SWIFT_FEATURE_ENABLE_BATCH_MODE,
         SWIFT_FEATURE_ENABLE_SKIP_FUNCTION_BODIES,
+        SWIFT_FEATURE_DISABLE_SYSTEM_INDEX,
         SWIFT_FEATURE_OBJC_LINK_FLAGS,
         SWIFT_FEATURE_OPT_USES_WMO,
         SWIFT_FEATURE_REMAP_XCODE_PATH,