Infer @_lifetime for mutating methods without experimental feature

Infer @_lifetime(self: copy self) for mutating methods without requiring the
experimental Lifetimes feature to be disabled. Treatment of mutating 'self' is
now consistent with other 'inout' parameters.

Example:

    extension MutableSpan {
      mutating func mutatingMethod() {...}
    }

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -481,6 +481,7 @@ class LifetimeDependenceChecker {
       // returning, 'depBuilder' contains any inferred dependencies. This does
       // not issue any diagnostics because using unsupported lifetime features
       // may generate a different diagnostic when the feature flag is disabled.
+      inferMutatingSelf();
       inferInoutParams();
 
       diagnoseMissingResultDependencies(