[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/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index a75b6c9..84e0ddd 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -316,7 +316,7 @@
 
 .. option:: -mmacosx-version-min=<version>
 
-  When building for Mac OS X, specify the minimum version supported by your
+  When building for macOS, specify the minimum version supported by your
   application.
 
 .. option:: -miphoneos-version-min