ASTReader: Copy input file offset data to avoid unaligned accesses

We interpret Blob as an array of uint64_t here, but there's no reason
to think that it has suitable alignment. Instead, read the data in in
an alignment-safe way and store it in a std::vector.

This fixes 225 test failures when clang is built with ubsan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240228 91177308-0d34-0410-b5e6-96231b3b80d8