[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/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index e2af5c6..ef3023e 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -15,7 +15,7 @@
     $ clang++ -stdlib=libc++ test.cpp
     $ clang++ -std=c++11 -stdlib=libc++ test.cpp
 
-On OS X and FreeBSD libc++ is the default standard library
+On macOS and FreeBSD libc++ is the default standard library
 and the ``-stdlib=libc++`` is not required.
 
 .. _alternate libcxx:
@@ -34,7 +34,7 @@
 The option ``-Wl,-rpath,<libcxx-install-prefix>/lib`` adds a runtime library
 search path. Meaning that the systems dynamic linker will look for libc++ in
 ``<libcxx-install-prefix>/lib`` whenever the program is run. Alternatively the
-environment variable ``LD_LIBRARY_PATH`` (``DYLD_LIBRARY_PATH`` on OS X) can
+environment variable ``LD_LIBRARY_PATH`` (``DYLD_LIBRARY_PATH`` on macOS) can
 be used to change the dynamic linkers search paths after a program is compiled.
 
 An example of using ``LD_LIBRARY_PATH``: