Add Target.cppTypeMangle hook for vendor types.

[ibuclaw]

This patch adds Target.cppMangleType, which returns a pointer to statically allocated string containing the target/vendor-specific mangling for the given type.

While this could apply for any type, have only added it to the basic, vector, and struct type visitor passes for now. DMD does nothing with this, however these specific places should catch all of the following:

• real may be a __float128 type mangled as g (alpha, ia64, ppc, systemz, sparc, x86)
• real may be mangled as a vendor type U10__float128 (ppc) or u9__float80 (ia64)
• va_list may be mangled as if it is in the "std" namespace St9__va_list (arm, aarch64)
• __vector() may differ depending on the default C++ ABI Dv (abi >= 4)

This replaces Target.realislongdouble.

See https://github.com/dlang/dmd/pull/6887/files?w=1 to read diff without whitespace changes.

Fixes the frontend part of bug reported against gdc: https://bugzilla.gdcproject.org/show_bug.cgi?id=247

FYI: @redstar / @klickverbot - this would affect you.

[UplinkCoder]

@ibuclaw vendor-type is a wired name.
How about platform-exclusiv-type ?

[ibuclaw]

@ibuclaw vendor-type is a wired name.

It's a perfectly reasonable name.

How about platform-exclusiv-type ?

Sounds too german. ☺️

[ibuclaw]

I forgot to mention that this also allows compilers to use internal information they have on the default C++ ABI. So if -fabi-version=4 or greater is the default, then you can return Dv instead of the current U8__vector. The reverse also applies when the frontend decides to switch C++ ABI to support >= 4.

[ibuclaw]

@redstar / @klickverbot - SYN.

[dnadlinger]

@ibuclaw: ACK. (… SYN ACK?)

[ibuclaw]

@klickverbot - ACK.