[Docs] Modernize references to macOS

Summary:
This updates all places in documentation that refer to "Mac OS X", "OS X", etc.
to instead use the modern name "macOS" when no specific version number is
mentioned.

If a specific version is mentioned, this attempts to use the OS name at the time
of that version:

* Mac OS X for 10.0 - 10.7
* OS X for 10.8 - 10.11
* macOS for 10.12 - present

Reviewers: JDevlieghere

Subscribers: mgorny, christof, arphaman, cfe-commits, lldb-commits, libcxx-commits, llvm-commits

Tags: #clang, #lldb, #libc, #llvm

Differential Revision: https://reviews.llvm.org/D62654

llvm-svn: 362113
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d45eaf9405c865cae0737a92d5ebce4e485106c3
diff --git a/docs/index.rst b/docs/index.rst
index 0fd3540..933d0fe 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -93,7 +93,7 @@
 ============ ==================== ============ ========================
 OS           Arch                 Compilers    ABI Library
 ============ ==================== ============ ========================
-Mac OS X     i386, x86_64         Clang, GCC   libc++abi
+macOS        i386, x86_64         Clang, GCC   libc++abi
 FreeBSD 10+  i386, x86_64, ARM    Clang, GCC   libcxxrt, libc++abi
 Linux        i386, x86_64         Clang, GCC   libc++abi
 ============ ==================== ============ ========================