[JITLoaderGDB] Set eTypeJIT for objects read from JIT descriptors
First part of a fix for JITed code debugging. This has been a regression from 5.0 to 6.0 and it's is still reproducible on current master: https://bugs.llvm.org/show_bug.cgi?id=36209
The address of the breakpoint site is corrupt: the 0x4 value we end up with, looks like an offset on a zero base address. When we parse the ELF section headers from the JIT descriptor, the load address for the text section we find in header.sh_addr is correct.
The bug manifests in VMAddressProvider::GetVMRange(const ELFSectionHeader &) (follow it from ObjectFileELF::CreateSections()). Here we think the object type was eTypeObjectFile and unleash some extra logic  which essentially overwrites the address with a zero value.
The object type is deduced from the ELF header's e_type in ObjectFileELF::CalculateType(). It never returns eTypeJIT, because the ELF header has no representation for it . Instead the in-memory ELF object states ET_REL, which leads to eTypeObjectFile. This is what we get from lli at least since 3.x. (Might it be better to write ET_EXEC on the JIT side instead? In fact, relocations were already applied at this point, so "Relocatable" is not quite exact.)
So, this patch proposes to set eTypeJIT explicitly whenever we read from a JIT descriptor. In ObjectFileELF::CreateSections() we can then call GetType(), which returns the explicit value or otherwise falls back to CalculateType().
LLDB then sets the breakpoint successfully. Next step: debug info.
Process 1056 stopped * thread #1, name = 'lli', stop reason = breakpoint 1.2 frame #0: 0x00007ffff7ff7000 JIT(0x3ba2030)`jitbp() JIT(0x3ba2030)`jitbp: -> 0x7ffff7ff7000 <+0>: pushq %rbp 0x7ffff7ff7001 <+1>: movq %rsp, %rbp 0x7ffff7ff7004 <+4>: movabsq $0x7ffff7ff6000, %rdi ; imm = 0x7FFFF7FF6000 0x7ffff7ff700e <+14>: movabsq $0x7ffff6697e80, %rcx ; imm = 0x7FFFF6697E80
 It was first introduced with https://reviews.llvm.org/D38142#change-lF6csxV8HdlL, which has also been the original breaking change. The code has changed a lot since then.
Reviewers: labath, JDevlieghere, bkoropoff, clayborg, espindola, alexshap, stella.stamenova
Reviewed By: labath, clayborg
Subscribers: probinson, emaste, aprantl, arichardson, MaskRay, AlexDenisov, yurydelendik, lldb-commits
Differential Revision: https://reviews.llvm.org/D61611