Update libc++ docs to include instructions for LIT.

Okay, so this actually does more than just that. I've rearranged most of
the information on the page to try to make it more helpful and flow
better.  Essentially, the differences between Mac and Linux, the various
ABI libraries, and in-tree versus out-of-tree builds were cluttering
things. To clean up, I've done the following:

 * Only describe the cmake process. buildit doesn't work out of the box
   on Linux, and we need to stop having duplicates for every process.
 * Use libc++abi for the default instructions. This works on the major
   platforms.
 * Describe both in-tree and out-of-tree builds. Previously it wasn't
   clear that in-tree builds were even possible for libc++.
 * Separate the documentation about using libc++ from that about
   building and testing libc++.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215358 91177308-0d34-0410-b5e6-96231b3b80d8
Add return statement to slice_array and mask_array assignment. Closes PR20614.

This patch just adds the required return statements to slice_array::operator=
and mask_array::operator=.

Tests were added to check that the return value is the same as the object assigned
to.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215414 91177308-0d34-0410-b5e6-96231b3b80d8
Change two tests to be less dependant on locales.

This patch removes the use of the "%c" specifier for getting/setting times.
The semantics of this specifier differ between linux and Mac. I don't believe
the use of this specifier was important to the test.

The following tests now pass on linux.
test/input.output/iostream.format/ext.manip/get_time.pass.cpp
test/input.output/iostream.format/ext.manip/put_time.pass.cpp

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215417 91177308-0d34-0410-b5e6-96231b3b80d8
Update libc++ docs to include instructions for LIT.

Okay, so this actually does more than just that. I've rearranged most of
the information on the page to try to make it more helpful and flow
better.  Essentially, the differences between Mac and Linux, the various
ABI libraries, and in-tree versus out-of-tree builds were cluttering
things. To clean up, I've done the following:

 * Only describe the cmake process. buildit doesn't work out of the box
   on Linux, and we need to stop having duplicates for every process.
 * Use libc++abi for the default instructions. This works on the major
   platforms.
 * Describe both in-tree and out-of-tree builds. Previously it wasn't
   clear that in-tree builds were even possible for libc++.
 * Separate the documentation about using libc++ from that about
   building and testing libc++.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215358 91177308-0d34-0410-b5e6-96231b3b80d8
Merge remote-tracking branch 'upstream/master'