[Windows] Separate elements in -print-search-dirs with semicolons

Path lists on windows should always be separated by semicolons, not
colons. Reuse llvm::sys::EnvPathSeparator for this purpose (as that's
also a path list that is separated in the same way).

Alternatively, this could just be a local ifdef _WIN32 in this function,
or generalizing the existing EnvPathSeparator to e.g. a
llvm::sys::path::PathListSeparator?

Differential Revision: https://reviews.llvm.org/D61121

llvm-svn: 359233