Reduce allocations in AnalyzerConfig.UnescapeSectionName (#80288)

* Reduce allocations in AnalyzerConfig.UnescapeSectionName

Just a simple switch to using a pooled StringBuilder. Not a huge allocator, but easy enough to switch.

Author: ToddGrun

src/Compilers/Core/Portable/CommandLine/AnalyzerConfig.SectionNameMatching.cs

@@ -123,7 +123,8 @@ public bool IsMatch(string s)
 
         internal static string UnescapeSectionName(string sectionName)
         {
-            var sb = new StringBuilder();
+            var pooledStrbuilder = PooledStringBuilder.GetInstance();
+            StringBuilder sb = pooledStrbuilder.Builder;
             SectionNameLexer lexer = new SectionNameLexer(sectionName);
             while (!lexer.IsDone)
             {
@@ -139,7 +140,7 @@ internal static string UnescapeSectionName(string sectionName)
                     throw ExceptionUtilities.UnexpectedValue(tokenKind);
                 }
             }
-            return sb.ToString();
+            return pooledStrbuilder.ToStringAndFree();
         }
 
         /// <summary>