Don't use libc's "char *basename(char *)" or "char *dirname(char *)" as they are not thread safe.

I switched the lldb_private::FileSpec code over to use "llvm::StringRef llvm::sys::path::filename(llvm::StringRef)" for basename() and "llvm::StringRef llvm::sys::path::parent_path(llvm::StringRef)" for dirname().

<rdar://problem/16870083>

llvm-svn: 209917