[Object, MachO] Cache parsed MachO header in MachOObjectFile. NFC.

Summary:
Avoid parsing object file each time MachOObjectFile::getHeader() is
called. Instead, cache the header in MachOObjectFile constructor, where
it's parsed anyway. In future, we must avoid constructing the object
at all if the header can't be parsed.

Test Plan: regression test suite.

Reviewers: rafael

Subscribers: llvm-commits
llvm-svn: 239075