Enforce __cplusplus >= 201703L on MSVC

Client code on MSVC *must* pass /Zc:__cplusplus when using Qt.
Otherwise, this makes Qt code that relies on feature-testing
macros a mess. For instance, in QTBUG-91117, we trip on this code:

  // C++ version guard is necessary: you may have the header,
  // but including it in pre-C++20 will cause an hard error
  #if __has_include(<bit>) && __cplusplus > 201703L
  #include <bit>
  #endif

  #if defined(__cpp_lib_bitops)
  // use some <bit> functionality
  #endif

The #define __cpp_lib_bitops should've come from the preceding include
directive, but there's another possibility: that it comes from
<version> (or some other similar header) included transitively,
when compiling in C++20 mode, and *without* a bumped __cplusplus.
Yes, that's an actual possibility on MSVC.

Then, since we did not include <bit> ourselves due to the __cplusplus
version check, using the functionality will cause a compile error.

We're not going to fix *every* post C++-17 feature detection macro
because of MSVC and feature-test shenanigans. It's time to require
compilers to tell us the truth about what they support.

Fixes: QTBUG-91117
Change-Id: I9d74f9d8b74b5ac35dce3528e7a2006746a00676
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/corelib/global/qglobal.h

@@ -86,6 +86,16 @@
 #include <QtCore/qprocessordetection.h>
 #include <QtCore/qcompilerdetection.h>
 
+// This could go to the very beginning of this file, but we're using compiler
+// detection, so it's here.
+#if defined(__cplusplus) && (__cplusplus < 201703L)
+#  ifdef Q_CC_MSVC
+#    error "Qt requires a C++17 compiler, and a suitable value for __cplusplus. On MSVC, you must pass the /Zc:__cplusplus option to the compiler."
+#  else
+#    error "Qt requires a C++17 compiler"
+#  endif
+#endif // __cplusplus
+
 #if defined (__ELF__)
 #  define Q_OF_ELF
 #endif