[mlir] Fix missing OpInterface docs newline

[seldridge]

Summary

Fix incorrect markdown generated by mlir-tblgen for an InterfaceMethod
that includes a body. Previously, this would cause the next method to
show up on the same line and produce incorrect markdown. Newlines would
only be added if the method did _not_ provide a body. E.g., previously
this was generating markdown like:

some function comment#### `next method`

This change makes this generate as:

some function comment

#### `next method`

Signed-off-by: Schuyler Eldridge <[e-mail]>

mlir/test/mlir-tblgen/op-interface.td

 // RUN: mlir-tblgen -gen-op-interface-decls -I %S/../../include %s | FileCheck %s --check-prefix=DECL
 // RUN: mlir-tblgen -gen-op-decls -I %S/../../include %s | FileCheck %s --check-prefix=OP_DECL
+// RUN: mlir-tblgen -gen-op-interface-docs -I %S/../../include %s | FileCheck %s --check-prefix=DOCS
 
 include "mlir/IR/OpBase.td"
 
 def ExtraShardDeclsInterface : OpInterface<"ExtraShardDeclsInterface"> {
   let extraSharedClassDeclaration = [{
     bool sharedMethodDeclaration() {
       return $_op.someOtherMethod();
     }
InterfaceMethod<
       /*desc=*/[{some function comment}],
       /*retTy=*/"int",
       /*methodName=*/"foo",
       /*args=*/(ins "int":$input)
     >,
     InterfaceMethod<
       /*desc=*/[{some function comment}],
       /*retTy=*/"int",
+      /*methodName=*/"body_foo",
+      /*args=*/(ins "int":$input),
+      /*body=*/[{ return 0; }]
+    >,
+    InterfaceMethod<
+      /*desc=*/[{some function comment}],
+      /*retTy=*/"int",
       /*methodName=*/"default_foo",
       /*args=*/(ins "int":$input),
       /*body=*/[{}],
       /*defaultBody=*/[{ return 0; }]
     >,
   ];
 }
 

 
 // OP_DECL-LABEL: class DeclareMethodsOp : public
 // OP_DECL: int foo(int input);
 // OP_DECL-NOT: int default_foo(int input);
 
 // OP_DECL-LABEL: class DeclareMethodsWithDefaultOp : public
 // OP_DECL: int foo(int input);
 // OP_DECL: int default_foo(int input);
+
+// DOCS-LABEL: {{^}}## TestOpInterface (`TestOpInterface`)
+// DOCS: some op interface description
+
+// DOCS: {{^}}### Methods:
+
+// DOCS: {{^}}#### `foo`
+// DOCS: some function comment
+
+// DOCS: {{^}}#### `body_foo`
+// DOCS: some function comment
+
+// DOCS: {{^}}#### `default_foo`
+// DOCS: some function comment

mlir/tools/mlir-tblgen/OpInterfacesGen.cpp

for (const auto &method : interface.getMethods()) {
     os << ");\n```\n";
 
     // Emit the description.
     if (auto description = method.getDescription())
       mlir::tblgen::emitDescription(*description, os);
 
     // If the body is not provided, this method must be provided by the user.
     if (!method.getBody())
-      os << "\nNOTE: This method *must* be implemented by the user.\n\n";
+      os << "\nNOTE: This method *must* be implemented by the user.";
+
+    os << "\n\n";
   }
 }
 
 bool InterfaceGenerator::emitInterfaceDocs() {
   os << "<!-- Autogenerated by mlir-tblgen; don't manually edit -->\n";
   os << "# " << interfaceBaseType << " definitions\n";
 
   for (const auto *def : defs)