[sanitizer_common] Use O_TRUNC for WrOnly access mode.

Summary: Otherwise if the file existed and was larger than the write size before the OpenFile call, the file will not be truncated and contain garbage in trailing bytes.

Reviewers: glider, kcc, vitalybuka

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48250

llvm-svn: 334881