Sketch out parser/IR support for OperationInst, and a new Instruction base
class.

Introduce an Identifier class to MLIRContext to represent uniqued identifiers,
introduce string literal support to the lexer, introducing parser and printer
support etc.

PiperOrigin-RevId: 202592007
diff --git a/include/mlir/IR/BasicBlock.h b/include/mlir/IR/BasicBlock.h
index e2de9d4..1858558 100644
--- a/include/mlir/IR/BasicBlock.h
+++ b/include/mlir/IR/BasicBlock.h
@@ -36,7 +36,10 @@
     return function;
   }
 
-  // TODO: bb arguments, instruction list.
+  // TODO: bb arguments
+
+  // TODO: Wrong representation.
+  std::vector<OperationInst*> instList;
 
   void setTerminator(TerminatorInst *inst) {
     terminator = inst;