[sanitizer] Cleanup ReadFileToVector and ReadFileToBuffer

Summary:
Added unit-test.
Fixed behavior of max_len argument.
Call read syscall with all available buffer, not just a page.

Reviewers: eugenis

Subscribers: kubamracek, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46618

llvm-svn: 334130