[dsymutil] Refactor BinaryHolder internals. NFC

Call a helper that resets all the internal state of the BinaryHolder
when we change the underlying memory buffer. Makes a followup patch
a tiny bit smaller.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243094 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed