Expose String.GetHashCode(StringComparison) with tests (#19443)

* Added String.GetHashCode(StringComparison) overload

* Added String.GetHashCode(StringComparison) tests

Author: jnm2

src/System.Runtime/ref/System.Runtime.cs

@@ -2030,6 +2030,7 @@ public void CopyTo(int sourceIndex, char[] destination, int destinationIndex, in
         public static string Format(string format, params object[] args) { throw null; }
         public System.CharEnumerator GetEnumerator() { throw null; }
         public override int GetHashCode() { throw null; }
+        public int GetHashCode(System.StringComparison comparisonType) { throw null; }
         public int IndexOf(char value) { throw null; }
         public int IndexOf(char value, int startIndex) { throw null; }
         public int IndexOf(char value, int startIndex, int count) { throw null; }

src/System.Runtime/src/ApiCompatBaseline.uap.txt

@@ -1,6 +1 @@
-Compat issues with assembly System.Runtime:
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.Char, System.Object[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.Char, System.String[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.String, System.Object[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.String, System.String[])' does not exist in the implementation but it does exist in the contract.
-Total Issues: 4
+Total Issues: 0

src/System.Runtime/src/ApiCompatBaseline.uapaot.txt

@@ -4,8 +4,5 @@ MembersMustExist : Member 'System.String.EndsWith(System.Char)' does not exist i
 MembersMustExist : Member 'System.String.Replace(System.String, System.String, System.Boolean, System.Globalization.CultureInfo)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'System.String.Replace(System.String, System.String, System.StringComparison)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'System.String.StartsWith(System.Char)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.Char, System.Object[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.Char, System.String[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.String, System.Object[])' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'System.Text.StringBuilder.AppendJoin(System.String, System.String[])' does not exist in the implementation but it does exist in the contract.
-Total Issues: 10
+MembersMustExist : Member 'System.String.GetHashCode(System.StringComparison)' does not exist in the implementation but it does exist in the contract.
+Total Issues: 7

src/System.Runtime/tests/System/StringTests.netcoreapp.cs

@@ -4,6 +4,7 @@
 
 using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using Xunit;
 
 namespace System.Tests
@@ -275,5 +276,32 @@ public void Replace_NoSuchStringComparison_ThrowsArgumentException(StringCompari
         {
             Assert.Throws<ArgumentException>("comparisonType", () => "abc".Replace("abc", "def", comparisonType));
         }
+
+
+        private static readonly StringComparison[] StringComparisons = (StringComparison[])Enum.GetValues(typeof(StringComparison));
+
+
+        public static IEnumerable<object[]> GetHashCode_StringComparison_Data => StringComparisons.Select(value => new object[] { value });
+
+        [Theory]
+        [MemberData(nameof(GetHashCode_StringComparison_Data))]
+        public static void GetHashCode_StringComparison(StringComparison comparisonType)
+        {
+            Assert.Equal(StringComparer.FromComparison(comparisonType).GetHashCode("abc"), "abc".GetHashCode(comparisonType));
+        }
+
+
+        public static IEnumerable<object[]> GetHashCode_NoSuchStringComparison_ThrowsArgumentException_Data => new[]
+        {
+            new object[] { StringComparisons.Min() - 1 },
+            new object[] { StringComparisons.Max() + 1 },
+        };
+
+        [Theory]
+        [MemberData(nameof(GetHashCode_NoSuchStringComparison_ThrowsArgumentException_Data))]
+        public static void GetHashCode_NoSuchStringComparison_ThrowsArgumentException(StringComparison comparisonType)
+        {
+            Assert.Throws<ArgumentException>("comparisonType", () => "abc".GetHashCode(comparisonType));
+        }
     }
 }