Update coding standards for include style

[zturner]

Summary

Per discussion on lldb-dev mailing list the other day.

[lattner]

LGTM!

llvm/trunk/docs/CodingStandards.rst

 header file), the `minimal list of #includes`_ required by the file should be
 listed.  We prefer these ``#include``\s to be listed in this order:
 
 .. _Main Module Header:
 .. _Local/Private Headers:
 
 #. Main Module Header
 #. Local/Private Headers
-#. ``llvm/...``
+#. LLVM project/subproject headers (``clang/...``, ``lldb/...``, ``llvm/...``, etc)
 #. System ``#include``\s
 
 and each category should be sorted lexicographically by the full path.
 
 The `Main Module Header`_ file applies to ``.cpp`` files which implement an
 interface defined by a ``.h`` file.  This ``#include`` should always be included
 **first** regardless of where it lives on the file system.  By including a
 header file first in the ``.cpp`` files that implement the interfaces, we ensure
 that the header does not have any hidden dependencies which are not explicitly
 ``#include``\d in the header, but should be. It is also a form of documentation
 in the ``.cpp`` file to indicate where the interfaces it implements are defined.
 
+LLVM project and subproject headers should be grouped from most specific to least
+specific, for the same reasons described above.  For example, LLDB depends on
+both clang and LLVM, and clang depends on LLVM.  So an LLDB source file should
+include ``lldb`` headers first, followed by ``clang`` headers, followed by
+``llvm`` headers, to reduce the possibility (for example) of an LLDB header
+accidentally picking up a missing include due to the previous inclusion of that
+header in the main source file or some earlier header file.  clang should
+similarly include its own headers before including llvm headers.  This rule
+applies to all LLVM subprojects.
+
 .. _fit into 80 columns:
 
 Source Code Width
 ^^^^^^^^^^^^^^^^^
 
 Write your code to fit within 80 columns of text.  This helps those of us who
 like to print out code and look at your code in an ``xterm`` without resizing
 it.