Modify (Large/Source)Text.ParseLineStart to specify an initial capacity for line count

src/Compilers/Core/Portable/Text/LargeText.cs

@@ -238,7 +238,9 @@ private SegmentedList<int> ParseLineStarts()
             var position = 0;
             var index = 0;
             var lastCr = -1;
-            var list = new SegmentedList<int>();
+            // Initial line capacity estimated at 64 chars / line. This value was obtained by
+            // looking at ratios in large files in the roslyn repo.
+            var list = new SegmentedList<int>(Length / 64);
 
             // The following loop goes through every character in the text. It is highly
             // performance critical, and thus inlines knowledge about common line breaks

src/Compilers/Core/Portable/Text/SourceText.cs

@@ -1049,7 +1049,9 @@ private SegmentedList<int> ParseLineStarts()
                 return [0];
             }
 
-            var lineStarts = new SegmentedList<int>()
+            // Initial line capacity estimated at 64 chars / line. This value was obtained by
+            // looking at ratios in large files in the roslyn repo.
+            var lineStarts = new SegmentedList<int>(Length / 64)
             {
                 0 // there is always the first line
             };