Replace InternalScopedBuffer<char> with InternalScopedString where applicable.

Summary: No functionality change.

Test Plan: make check-all

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6472

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@223164 91177308-0d34-0410-b5e6-96231b3b80d8