"main include" that will be the 1st include (category 0).

Because the clang-format visual studio extension does not pass the file
name and use the standard input, sort include cannot find a "main
include":

Testing fix on llvm\tools\clang\lib\Format\Format.cpp:
Original file:
  #include "clang/Format/Format.h"
  ...
  #include "clang/Basic/SourceManager.h"
  #include "clang/Lex/Lexer.h"

Without fix, selecting the includes and running visual studio
clang-format:
  ...
  #include "clang/Basic/SourceManager.h"
  #include "clang/Format/Format.h"
  #include "clang/Lex/Lexer.h"

With fix, selecting the includes and running visual studio clang-format:
  #include "clang/Format/Format.h"
  ...
  #include "clang/Basic/SourceManager.h"
  #include "clang/Lex/Lexer.h"

Test 2 with main header not at the start:
Original file:
  ...
  #include "clang/Format/Format.h"
  #include "clang/Basic/SourceManager.h"
  #include "clang/Lex/Lexer.h"

Without fix, selecting the includes and running visual studio
clang-format:
  ...
  #include "clang/Basic/SourceManager.h"
  #include "clang/Format/Format.h"
  #include "clang/Lex/Lexer.h"

With fix, selecting the includes and running visual studio clang-format:
  #include "clang/Format/Format.h"
  ...
  #include "clang/Basic/SourceManager.h"
  #include "clang/Lex/Lexer.h"

Patch by Jean-Philippe Dufraigne, thank you.
Review: http://reviews.llvm.org/D16524

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260378 91177308-0d34-0410-b5e6-96231b3b80d8