Updated AllWellKnownTypes tests

src/Compilers/CSharp/Test/Symbol/Symbols/MissingSpecialMember.cs

@@ -561,6 +561,7 @@ public void AllWellKnownTypes()
                     case WellKnownType.System_Runtime_CompilerServices_IsReadOnlyAttribute:
                     case WellKnownType.System_Runtime_CompilerServices_IsByRefLikeAttribute:
                     case WellKnownType.System_Span_T:
+                    case WellKnownType.System_ReadOnlySpan_T:
                     // Not yet in the platform.
                     case WellKnownType.Microsoft_CodeAnalysis_Runtime_Instrumentation:
                         // Not always available.

src/Compilers/VisualBasic/Test/Symbol/SymbolsTests/WellKnownTypeValidationTests.vb

@@ -502,7 +502,8 @@ End Namespace
                         Continue For
                     Case WellKnownType.System_FormattableString,
                          WellKnownType.System_Runtime_CompilerServices_FormattableStringFactory,
-                         WellKnownType.System_Span_T
+                         WellKnownType.System_Span_T,
+                         WellKnownType.System_ReadOnlySpan_T
                         ' Not available on all platforms.
                         Continue For
                     Case WellKnownType.ExtSentinel
@@ -540,7 +541,8 @@ End Namespace
                         Continue For
                     Case WellKnownType.System_FormattableString,
                          WellKnownType.System_Runtime_CompilerServices_FormattableStringFactory,
-                         WellKnownType.System_Span_T
+                         WellKnownType.System_Span_T,
+                         WellKnownType.System_ReadOnlySpan_T
                         ' Not available on all platforms.
                         Continue For
                     Case WellKnownType.ExtSentinel