Implement SynthesizedBackingFieldSymbolBase.TryGetFirstLocation (#82679)

This avoid the allocations done in the Symbol.TryGetFirstLocation usage
of the Locations property.

This accounts for 0.7% of allocations in a scenario in the
ComplietionInCohosting razor speedometer test.

<img width="1683" height="505" alt="image"
src="https://github.com/user-attachments/assets/8b103bf1-5125-41e8-a41a-191067791806"
/>

Author: ToddGrun

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedBackingFieldSymbol.cs

@@ -119,6 +119,9 @@ public override Symbol AssociatedSymbol
         public override ImmutableArray<Location> Locations
             => _property.Locations;
 
+        public override Location TryGetFirstLocation()
+            => _property.TryGetFirstLocation();
+
         public override RefKind RefKind => _property.RefKind;
 
         public override ImmutableArray<CustomModifier> RefCustomModifiers => _property.RefCustomModifiers;