Parse operations in ML functions. Add builder class for ML functions.

Refactors operation parsing to share functionality between CFG and ML functions. ML function construction now goes through a builder, similar to the way it is done for
CFG functions.

PiperOrigin-RevId: 204779279
diff --git a/test/IR/parser-errors.mlir b/test/IR/parser-errors.mlir
index ff03b51..7efff5b 100644
--- a/test/IR/parser-errors.mlir
+++ b/test/IR/parser-errors.mlir
@@ -130,7 +130,7 @@
 // -----
 
 mlfunc @non_statement() {
-  asd   // expected-error {{expected statement}}
+  asd   // expected-error {{expected operation name in quotes}}
 }
 
 // -----