clang-format: Add preprocessor directive indentation

Summary:
This is an implementation for [bug 17362](https://bugs.llvm.org/attachment.cgi?bugid=17362) which adds support for indenting preprocessor statements inside if/ifdef/endif. This takes previous work from fmauch (https://github.com/fmauch/clang/tree/preprocessor_indent) and makes it into a full feature.
The context of this patch is that I'm a VMware intern, and I implemented this because VMware needs the feature. As such, some decisions were made based on what VMware wants, and I would appreciate suggestions on expanding this if necessary to use-cases other people may want.

This adds a new enum config option, `IndentPPDirectives`. Values are:

* `PPDIS_None` (in config: `None`):
```
    #if FOO
    #if BAR
    #include <foo>
    #endif
    #endif
```
* `PPDIS_AfterHash` (in config: `AfterHash`):
```
    #if FOO
    #  if BAR
    #    include <foo>
    #  endif
    #endif
```
This is meant to work whether spaces or tabs are used for indentation. Preprocessor indentation is independent of indentation for non-preprocessor lines.

Preprocessor indentation also attempts to ignore include guards with the checks:
1. Include guards cover the entire file
2. Include guards don't have `#else`
3. Include guards begin with
```
#ifndef <var>
#define <var>
```

This patch allows `UnwrappedLineParser::PPBranchLevel` to be decremented to -1 (the initial value is -1) so the variable can be used for indent tracking.

Defects:
* This patch does not handle the case where there's code between the `#ifndef` and `#define` but all other conditions hold. This is because when the #define line is parsed, `UnwrappedLineParser::Lines` doesn't hold the previous code line yet, so we can't detect it. This is out of the scope of this patch.

* This patch does not handle cases where legitimate lines may be outside an include guard. Examples are `#pragma once` and `#pragma GCC diagnostic`, or anything else that does not change the meaning of the file if it's included multiple times.

* This does not detect when there is a single non-preprocessor line in front of an include-guard-like structure where other conditions hold because `ScopedLineState` hides the line.

* Preprocessor indentation throws off `TokenAnnotator::setCommentLineLevels` so the indentation of comments immediately before indented preprocessor lines is toggled on each run. Fixing this issue appears to be a major change and too much complexity for this patch.

Contributed by @euhlmann!

Reviewers: djasper, klimek, krasimir

Reviewed By: djasper, krasimir

Subscribers: krasimir, mzeren-vmw, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D35955

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312125 91177308-0d34-0410-b5e6-96231b3b80d8