Remove use of non ASCII characters in literals.

We would prefer that we don't use non ASCII characters in either string
or character literals.  In theory, every editor and tool would ensure
that these characters are not translated to some random encoding or
otherwise become corrupted.  In practice, we're just going to err on the
side of caution.

src/System.Reflection.Metadata/tests/Utilities/MemoryBlockTests.cs

@@ -73,16 +73,16 @@ public unsafe void DefaultDecodingFallbackMatchesBcl()
             fixed (byte* ptr = (buffer = new byte[] { 0xC0 }))
             {
                 string s = new MemoryBlock(ptr, buffer.Length).PeekUtf8NullTerminated(0, null, decoder, out bytesRead);
-                Assert.Equal("�", new MemoryBlock(ptr, buffer.Length).PeekUtf8NullTerminated(0, null, decoder, out bytesRead));
+                Assert.Equal("\uFFFD", new MemoryBlock(ptr, buffer.Length).PeekUtf8NullTerminated(0, null, decoder, out bytesRead));
                 Assert.Equal(s, Encoding.UTF8.GetString(buffer));
                 Assert.Equal(buffer.Length, bytesRead);
 
                 s = new MemoryBlock(ptr, buffer.Length).PeekUtf8NullTerminated(0, Encoding.UTF8.GetBytes("Hello"), decoder, out bytesRead);
-                Assert.Equal("Hello�", s);
+                Assert.Equal("Hello\uFFFD", s);
                 Assert.Equal(s, "Hello" + Encoding.UTF8.GetString(buffer));
                 Assert.Equal(buffer.Length, bytesRead);
 
-                Assert.Equal("�", new MemoryBlock(ptr, buffer.Length).PeekUtf8(0, buffer.Length));
+                Assert.Equal("\uFFFD", new MemoryBlock(ptr, buffer.Length).PeekUtf8(0, buffer.Length));
             }
 
             // overlong encoding
@@ -102,7 +102,7 @@ public unsafe void DecodingSuccessMatchesBcl()
             var utf8 = Encoding.GetEncoding("utf-8", EncoderFallback.ExceptionFallback, DecoderFallback.ExceptionFallback);
             byte[] buffer;
             int bytesRead;
-            string str = "你好. Comment ça va?";
+            string str = "\u4F60\u597D. Comment \u00E7a va?";
 
             var decoder = MetadataStringDecoder.DefaultUTF8;
 
@@ -142,9 +142,9 @@ public unsafe void LightUpTrickFromDifferentAssemblyWorks()
             Assert.Throws<ArgumentOutOfRangeException>(() => decoder.GetString((byte*)1, -1));
 
             byte[] bytes;
-            fixed (byte* ptr = (bytes = Encoding.Unicode.GetBytes("Ça marche très bien.")))
+            fixed (byte* ptr = (bytes = Encoding.Unicode.GetBytes("\u00C7a marche tr\u00E8s bien.")))
             {
-                Assert.Equal("Ça marche très bien.", decoder.GetString(ptr, bytes.Length));
+                Assert.Equal("\u00C7a marche tr\u00E8s bien.", decoder.GetString(ptr, bytes.Length));
             }
         }