[libcxx] Allow use of ShTest in libc++ tests along with other changes.

Summary:
This patch allows the use of LIT's ShTest format in the libc++ test suite. ShTests have the suffix '.sh.cpp'. It also introduces a series of other changes. These changes are:

- More functionality including parsing test metadata has been moved into LIT.
- LibcxxTestFormat now supports multi-part suffixes.
- the `CXXCompiler` functionality has been used to shrink the size of LibcxxTestFormat. 
- The recursive loading of the site config has been turned into `libcxx.test.config.loadSiteConfig` so it can be used with libc++abi.
- Temporary files are now created in the build directory of libc++. This follows how it is down in ShTest.
- `not.py` was added as a utility executable that mirrors the functionality of LLVM's `not` executable. 
- The first ShTest test was added under test/libcxx/double_include.sh.cpp


Reviewers: jroelofs, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7073

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@226844 91177308-0d34-0410-b5e6-96231b3b80d8

www/lit_usage.html

@@ -131,6 +131,16 @@ Note: This does not use the installed headers.
 </blockquote>
 </p>
 
+<p>
+<h3 class="lit-option">use_lit_shell=&lt;bool&gt;</h3>
+<blockquote class="lit-option-desc">
+Enable or disable the use of LIT's internal shell in ShTests. If the enviroment
+variable <code>LIT_USE_INTERNAL_SHELL</code> is present then that is used as the
+default value. Otherwise the default value is <code>True</code> on Windows and
+<code>False</code> on every other platform.
+</blockquote>
+</p>
+
 <p>
 <h3 class="lit-option">no_default_flags=&lt;bool&gt;</h3>
 <blockquote class="lit-option-desc">