[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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@362113 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/sphinx_intro.rst b/docs/sphinx_intro.rst
index 6bb9816..b671cdc 100644
--- a/docs/sphinx_intro.rst
+++ b/docs/sphinx_intro.rst
@@ -43,8 +43,8 @@
     Use your distribution's standard package management tool to install it,
     i.e., ``apt-get install easy_install`` or ``yum install easy_install``.
 
-  Mac OS X
-    All modern Mac OS X systems come with ``easy_install`` as part of the base
+  macOS
+    All modern macOS systems come with ``easy_install`` as part of the base
     system.
 
   Windows