qcompilerdetection.h: add Q_COMPILER_MANGLES_ACCESS_SPECIFIER

This is similar to Q_COMPILER_MANGLES_RETURN_TYPE, except that the
other such compiler, SunPro CC, at least by my cursory reading of

https://archive.org/download/SunWorkshopVol5No1/Sun%20WorkShop%E2%84%A2%20for%20Solaris%202.x%20Volume%205%20Number%201.iso/SPROmrcpl%2Freloc%2FSUNWspro%2FSC4.2%2FREADMEs%2Fmangling.ps

doesn't appear to mangle the access specifier. So it's only MSVC. It's
still better to have a properly-named macro for this than to work with
Q_CC_MSVC and, hopefully, an explanatory code comment.

May come in handy to maintain BC when we find the need to change an
access specifier in the future. The original use-case, in QtPositioning,
was fixed differently. If nothing else, let's have it to raise awareness
of the issue.

Pick-to: 6.5
Change-Id: Ia8b789d2713ec19487a21c6bb0a12cf285e6ba83
Reviewed-by: Fabian Kosmale <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>

src/corelib/global/qcompilerdetection.h

@@ -71,6 +71,7 @@
 #  endif
 #  define Q_OUTOFLINE_TEMPLATE inline
 #  define Q_COMPILER_MANGLES_RETURN_TYPE
+#  define Q_COMPILER_MANGLES_ACCESS_SPECIFIER
 #  define Q_FUNC_INFO __FUNCSIG__
 #  define Q_ASSUME_IMPL(expr) __assume(expr)
 #  define Q_UNREACHABLE_IMPL() __assume(0)