[LLDB] Remove a now redundant windows specific workaround

vsnprintf(NULL, 0, ...) works for measuring the needed string
size on all supported Windows variants; it's supported since
at least MSVC 2015 (and LLVM requires a newer version than that),
and works on both msvcrt.dll (since at least XP) and UCRT with MinGW.

The previous use of ifdefs was wrong as well, as __MINGW64__ only is
defined for 64 bit targets, and the define without trailing
underscores is never defined automatically (neither by clang nor
by gcc).

Differential Revision: https://reviews.llvm.org/D67861

llvm-svn: 372591