stdlib: remove a wrong internal check for COW array mutation

Don't complain if the empty array buffer singleton is set to mutable.
This can happen if reserveCapacity is called with a 0-capacity for an empty array.
In this case the resulting array is still the empty array singleton.
For the compiler, it's safe to treat the empty array singleton as "mutable", because there is never anything written to an array with zero capacity:
all mutating array operations do any form of capacity/size > 0 check in addition to the uniqueness check.

Unfortunately the empty array singleton sometimes needs such special handling with is not obvious (not to say hacky).

This wrong check only fired in very specific optimization scenarios, where the inliner and the COW optimizations must play together in a certain way.
I could not find an isolated test case for this problem.

rdar://problem/71107908

Author: eeckstein

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -470,18 +470,17 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
     nonmutating set {
       if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
         if (_COWChecksEnabled()) {
-          if newValue {
-            if capacity > 0 {
-              let wasImmutable = _swift_setImmutableCOWBuffer(_storage, true)
+          // Make sure to not modify the empty array singleton (which has a
+          // capacity of 0).
+          if capacity > 0 {
+            let wasImmutable = _swift_setImmutableCOWBuffer(_storage, newValue)
+            if newValue {
               _internalInvariant(!wasImmutable,
                 "re-setting immutable array buffer to immutable")
+            } else {
+              _internalInvariant(wasImmutable,
+                "re-setting mutable array buffer to mutable")
             }
-          } else {
-            _internalInvariant(capacity > 0,
-              "setting empty array buffer to mutable")
-             let wasImmutable = _swift_setImmutableCOWBuffer(_storage, false)
-            _internalInvariant(wasImmutable,
-              "re-setting mutable array buffer to mutable")
           }
         }
       }