[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/TestingGuide.rst b/llvm/docs/TestingGuide.rst
index 949fecf..fed9a6b 100644
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -511,7 +511,7 @@
    The suffix for the host platforms shared library files. This includes the
    period as the first character.
 
-   Example: ``.so`` (Linux), ``.dylib`` (OS X), ``.dll`` (Windows)
+   Example: ``.so`` (Linux), ``.dylib`` (macOS), ``.dll`` (Windows)
 
 ``%exeext``
    The suffix for the host platforms executable files. This includes the