Implement parser/IR support for CFG functions, basic blocks and return instruction.

This is pretty much minimal scaffolding for this step.  Basic block arguments,
instructions, other terminators, a proper IR representation for
blocks/instructions, etc are all coming.

PiperOrigin-RevId: 201826439
diff --git a/lib/Parser/Token.h b/lib/Parser/Token.h
index 36b1abd..c8f856c 100644
--- a/lib/Parser/Token.h
+++ b/lib/Parser/Token.h
@@ -40,10 +40,12 @@
 
     // Punctuation.
     arrow,              // ->
+    colon,              // :
     comma,              // ,
     question,           // ?
     questionquestion,   // ??
     l_paren, r_paren,   // ( )
+    l_brace, r_brace,   // { }
     less, greater,      // < >
     // TODO: More punctuation.
 
@@ -62,6 +64,7 @@
     kw_int,
     kw_memref,
     kw_mlfunc,
+    kw_return,
     kw_tensor,
     kw_vector,
   };