DebugInfo compression: Enable compression before any sections are created.

For those playing at home this produced some fairly subtle behavior. The
sections created in InitMCObjectFileInfo were created without compressed
debug info (a mistake, but not necessarily /broken). Since these
sections were almost always referenced by the existing MCSection object,
this almost worked.

This got weird when we got to handling the relocations for a section.
See ELFObjectWriter::WriteSection where we compute the true section for
a relocation section by simply stripping the ".rela" prefix and then
looking up that section - doing so hit the compression codepath, looked
up .zdebug_blah and found a newly constructed empty section... thus,
things got weird.

This is untestable without a cross-project test (let me know if people
would prefer that to no testing).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205261 91177308-0d34-0410-b5e6-96231b3b80d8
Warn when requesting compress-debug-sections and zlib is not available

Another shot in the dark, since I do have zlib installed. Will be
watching the bots for fallout.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205265 91177308-0d34-0410-b5e6-96231b3b80d8
Comment necessity of early initialization

Code review feedback from Eric Christopher on r204261.

I didn't want to go into too much detail (the revision history should
provide the full stuff) - but I can add more if that's preferred.

Also moved this up to right by the construction of the MCAsmInfo so
there's less chance that other things might sneak in in between.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205267 91177308-0d34-0410-b5e6-96231b3b80d8