[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
diff --git a/llvm/docs/CommandGuide/llvm-ar.rst b/llvm/docs/CommandGuide/llvm-ar.rst
index d3ee993..d10ec91 100644
--- a/llvm/docs/CommandGuide/llvm-ar.rst
+++ b/llvm/docs/CommandGuide/llvm-ar.rst
@@ -229,7 +229,7 @@
 
 The **llvm-ar** utility is intended to provide a superset of the IEEE Std 1003.2
 (POSIX.2) functionality for ``ar``. **llvm-ar** can read both SVR4 and BSD4.4 (or
-Mac OS X) archives. If the ``f`` modifier is given to the ``x`` or ``r`` operations
+macOS) archives. If the ``f`` modifier is given to the ``x`` or ``r`` operations
 then **llvm-ar** will write SVR4 compatible archives. Without this modifier,
 **llvm-ar** will write BSD4.4 compatible archives that have long names
 immediately after the header and indicated using the "#1/ddd" notation for the
@@ -240,7 +240,7 @@
 -----------
 
 
-The file format for LLVM Archive files is similar to that of BSD 4.4 or Mac OSX
+The file format for LLVM Archive files is similar to that of BSD 4.4 or macOS
 archive files. In fact, except for the symbol table, the ``ar`` commands on those
 operating systems should be able to read LLVM archive files. The details of the
 file format follow.