fix: Make functions in LambdaSupport.h purely virtual to avoid linking errors in debug builds

Some virtual functions in `LambdaSupport.h` are meant to be purely
virtual, but lack a `= 0` in their signature. In debug builds, this
causes the linker to look for a default implementation, resulting in a
linker error.

This patch marks the virtual functions as purely virtual, resolving
the linker issues in debug builds.

compiler/include/concretelang/Support/LambdaSupport.h

@@ -249,7 +249,7 @@ public:
   /// Compile the mlir program and produces a compilation result if succeed.
   llvm::Expected<std::unique_ptr<CompilationResult>> virtual compile(
       llvm::SourceMgr &program,
-      CompilationOptions options = CompilationOptions("main"));
+      CompilationOptions options = CompilationOptions("main")) = 0;
 
   llvm::Expected<std::unique_ptr<CompilationResult>>
   compile(llvm::StringRef program,
@@ -266,15 +266,16 @@ public:
   }
 
   /// Load the server lambda from the compilation result.
-  llvm::Expected<Lambda> virtual loadServerLambda(CompilationResult &result);
+  llvm::Expected<Lambda> virtual loadServerLambda(
+      CompilationResult &result) = 0;
 
   /// Load the client parameters from the compilation result.
   llvm::Expected<clientlib::ClientParameters> virtual loadClientParameters(
-      CompilationResult &result);
+      CompilationResult &result) = 0;
 
   /// Call the lambda with the public arguments.
   llvm::Expected<std::unique_ptr<clientlib::PublicResult>> virtual serverCall(
-      Lambda lambda, clientlib::PublicArguments &args);
+      Lambda lambda, clientlib::PublicArguments &args) = 0;
 
   /// Build the client KeySet from the client parameters.
   static llvm::Expected<std::unique_ptr<clientlib::KeySet>>