Deduplicate capability features

Copybara Import from #344

BEGIN_PUBLIC
Deduplicate capability features (#344)

Under the covers, features are used to communicate tool capabilities to the underlying code.  If multiple tools provide the same capabilities, this ends up generating duplicate features and failing.

C++ and C tools is a really easy example, and they both have "supports_dynamic_linker" as a capability very reasonably.

Closes #344
END_PUBLIC

COPYBARA_INTEGRATE_REVIEW=#344 from AustinSchuh:dedup_capabilities 74bc316
PiperOrigin-RevId: 782686204
Change-Id: Ic3575c8e2f873d8161812da508161ed485672135

Author: AustinSchuh

cc/toolchains/impl/toolchain_config_info.bzl

@@ -119,7 +119,7 @@ def _validate_feature(self, known_features, fail):
 def _validate_toolchain(self, fail = fail):
     capabilities = []
     for tool in self.tool_map.configs.values():
-        capabilities.extend([cap.feature for cap in tool.capabilities])
+        capabilities.extend([cap.feature for cap in tool.capabilities if cap.feature not in capabilities])
     known_features = _get_known_features(self.features, capabilities, fail = fail)
 
     for feature in self.features: