[UBSan] Improve bufferization of UBSan error reports.

Summary:
Use InternalScopedString more extensively. This reduces the number of
write() syscalls, and reduces the chance that UBSan output will be
mixed with program output.

Reviewers: vitalybuka

Subscribers: kcc, llvm-commits

Differential Revision: http://reviews.llvm.org/D18068

llvm-svn: 263176