[MLIR] add entry block to MLIR grammar.

[bollu]

Summary

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

mlir/docs/LangRef.md

 Regions do not have a name or an address, only the blocks contained in a region
 do. Regions must be contained within operations and have no type or attributes.
 The first block in the region is a special block called the 'entry block'. The
 arguments to the entry block are also the arguments of the region itself. The
 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
 has additional semantic restrictions that other types of regions may not have.
 For example, in a function body, block terminators must either branch to a
 different block, or return from a function where the types of the `return`
 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
 reference, i.e. branch to, a block which is not in the same region as the source
 of the reference, i.e. a terminator operation. Similarly, regions provides a
 natural scoping for value visibility: values defined in a region don't escape to
 the enclosing region, if any. By default, operations inside a region can
 reference values defined outside of the region whenever it would have been legal