Compiler: Avoid allocating strings in the RazorHtmlWriter (#11975)

This pull request is essentially a rewrite of `RazorHtmlWriter` with the
following changes:

- Simply span tracking for source mappings.
- Avoid allocating strings when writing whitespace to the `CodeWriter`
and reduce the number of writes.
- Avoid allocating strings when writing C# placeholder content (i.e.
`~~~` and `/*~~~~~~~*/`).
- Avoid allocating delegates to base visitor methods.
- Fix a bug this change uncovered in the `CodeWriter.Reader.Read(char[],
int, int)` implementation that can cause the `SourceText` produced to be
one character shorter than expected.

I recommend reviewing commit-by-commit.

----
CI Build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2736836&view=results
Test Insertion:
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/646317
Toolset Run:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2736837&view=results

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/CodeGeneration/CodeWriter.cs

@@ -415,7 +415,7 @@ public override int Read(char[] buffer, int index, int count)
 
             if (_page is null)
             {
-                return -1;
+                return 0;
             }
 
             var destination = buffer.AsSpan(index, count);
@@ -435,6 +435,12 @@ public override int Read(char[] buffer, int index, int count)
 
                 foreach (var chunk in chunks)
                 {
+                    if (destination.IsEmpty)
+                    {
+                        // If we have no more space in the destination, we're done.
+                        break;
+                    }
+
                     var source = chunk.Span;
 
                     // Slice if the first chunk is partial. Note that this only occurs for the first chunk.
@@ -461,16 +467,15 @@ public override int Read(char[] buffer, int index, int count)
                         charIndex = 0;
                     }
 
+                    if (source.IsEmpty)
+                    {
+                        continue;
+                    }
+
                     source.CopyTo(destination);
                     destination = destination[source.Length..];
 
                     charsWritten += source.Length;
-
-                    // Break if we are done writing. chunkIndex and charIndex should have their correct values at this point.
-                    if (destination.IsEmpty)
-                    {
-                        break;
-                    }
                 }
 
                 if (destination.IsEmpty)
@@ -497,6 +502,8 @@ public override int Read(char[] buffer, int index, int count)
                 _charIndex = -1;
             }
 
+            _remainingLength -= charsWritten;
+
             return charsWritten;
         }
 
@@ -551,6 +558,7 @@ public override string ReadToEnd()
             _page = null;
             _chunkIndex = -1;
             _charIndex = 1;
+            _remainingLength = 0;
 
             return result;
         }