[PseudoTerminal][NFC] Use llvm errno helpers

Summary:
LLVM provide (str)errno helpers, so convert code to use it.

Also fixes warning:
/home/xbolva00/LLVM/llvm/tools/lldb/source/Host/common/PseudoTerminal.cpp:248:25: warning: ignoring return value of ‘char* strerror_r(int, char*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]
             ::strerror_r(errno, error_str, error_len);

Reviewers: JDevlieghere

Reviewed By: JDevlieghere

Subscribers: abidh, lldb-commits

Differential Revision: https://reviews.llvm.org/D51591

llvm-svn: 341320