Add missing titles to few docs to make sure they're rendered in docs.

docs/source/doc/llvm.core.Constant.rst

@@ -4,6 +4,9 @@
 | title: Constant (llvm.core)   |
 +-------------------------------+
 
+llvm.core.Constant
+==================
+
 ``Constant``-s represents constants that appear within the code. The
 values of such objects are known at creation time. Constants can be
 created from Python constants. A constant expression is also a constant

docs/source/doc/llvm.core.GlobalVariable.rst

@@ -4,6 +4,9 @@
 | title: GlobalVariable (llvm.core)   |
 +-------------------------------------+
 
+llvm.core.GlobalVariable
+========================
+
 Global variables (``llvm.core.GlobalVariable``) are subclasses of
 `llvm.core.GlobalValue <llvm.core.GlobalValue.html>`_ and represent
 module-level variables. These can have optional initializers and can be

docs/source/doc/llvm.core.Module.rst

@@ -4,11 +4,13 @@
 | title: Module (llvm.core)   |
 +-----------------------------+
 
+llvm.core.Module
+================
+
 Modules are top-level container objects. You need to create a module
 object first, before you can add global variables, aliases or functions.
 Modules are created using the static method ``Module.new``:
 
-
 .. code-block:: python
 
    #!/usr/bin/env python

docs/source/doc/llvm.core.StructType.rst

@@ -0,0 +1,3 @@
+llvm.core.StructType
+====================
+