MSVC C5046 warning is unavailable in MSVC 2015.

Per the MSVC documentation the warning is new as of Visual Studio 2017,
version 15.8.
https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c5046?view=vs-2019

GTest users building on MSVC 2015 or older versions of 2017 will, when
C4616 is enabled, see a warning like:

[...]gtest-matchers.h(53): error C2220: warning treated as error - no 'object' file generated
[...]gtest-matchers.h(53): warning C4619: #pragma warning: there is no warning number '5046'

Guard the mention of 5046 by an _MSC_VER check. VS2017 15.8 corresponds
to an _MSC_VER of 1915.
https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=vs-2019