[MLIR] add entry block to MLIR grammar.

The MLIR parser allows regions to have an unnamed entry block.
Make this explicit in the language grammar.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D119950

mlir/docs/LangRef.md

@@ -443,7 +443,8 @@ entry block cannot be listed as a successor of any other block. The syntax for a
 region is as follows:
 
 ```
-region ::= `{` block* `}`
+region      ::= `{` entry-block? block* `}`
+entry-block ::= operation+
 ```
 
 A function body is an example of a region: it consists of a CFG of blocks and
@@ -454,6 +455,11 @@ arguments must match the result types of the function signature. Similarly, the
 function arguments must match the types and count of the region arguments. In
 general, operations with regions can define these correspondences arbitrarily.
 
+An *entry block* is a block with no label and no arguments that may occur at
+the beginning of a region. It enables a common pattern of using a region to
+open a new scope.
+
+
 ### Value Scoping
 
 Regions provide hierarchical encapsulation of programs: it is impossible to