Bitcode: Introduce a BitcodeFileContents data type. NFCI.

This data type includes the contents of a bitcode file.
Right now a bitcode file can only contain modules, but
a later change will add a symbol table.

Differential Revision: https://reviews.llvm.org/D33969

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305019 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed