[LLDB] Avoid warnings about redefining posix mode defines on MinGW

Since these defines were added in LLVM SVN r189364 in 2013,
mingw-w64 got defines for S_I?GRP, S_IRWXG, S_I?OTH and S_IRWXO
in 2015.

Also change the existing defined(_MSC_VER) into ifndef S_IRUSR, for
consistency.

Differential Revision: https://reviews.llvm.org/D67910

llvm-svn: 372700