VS 2019 16.3 deprecates <experimental/filesystem>. (#6968)

VS 2019 16.3 will contain a couple of source-breaking changes:

* <experimental/filesystem> will be deprecated via an
impossible-to-miss preprocessor "#error The <experimental/filesystem>
header providing std::experimental::filesystem is deprecated by
Microsoft and will be REMOVED. It is superseded by the C++17
<filesystem> header providing std::filesystem. You can define
_SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING to acknowledge
that you have received this warning."

* <filesystem> will no longer include <experimental/filesystem>.

In the long term, I believe that vcpkg should detect when it's being
built with VS 2017 15.7 or newer, compile in C++17 mode, include
<filesystem>, and use std::filesystem. (Activating this for VS 2019 16.0
or newer would also be reasonable.) Similarly for other toolsets
supporting std::filesystem.

In the short term, this commit makes vcpkg compatible with the upcoming
deprecation. First, we need to define the silencing macro before
including the appropriate header. I've chosen to define it
unconditionally (without checking for platform or version), since it
has no effect for other platforms or versions. Second, we need to deal
with <filesystem> no longer including <experimental/filesystem>.
I verified that VS 2015 Update 3 contained <experimental/filesystem>
(back then, it simply included the <filesystem> header, where the
experimental implementation was defined; this was later reorganized).
Therefore, all of vcpkg's supported MSVC toolsets have
<experimental/filesystem>, so we can simply always include it.

I've verified that this builds with both VS 2015 Update 3 and
VS 2019 16.1.3 (the current production version).

toolsrc/include/pch.h

@@ -37,11 +37,8 @@
 #include <cstdarg>
 #include <cstddef>
 #include <cstdint>
-#if defined(_WIN32)
-#include <filesystem>
-#else
+#define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING
 #include <experimental/filesystem>
-#endif
 #include <cstring>
 #include <fstream>
 #include <functional>

toolsrc/include/vcpkg/base/files.h

@@ -2,11 +2,8 @@
 
 #include <vcpkg/base/expected.h>
 
-#if defined(_WIN32)
-#include <filesystem>
-#else
+#define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING
 #include <experimental/filesystem>
-#endif
 
 namespace fs
 {