[rfc][gold plugin] Fix pr19901

[Rafael Espíndola]

I am going on vacations tonight, so I don't think I will have time to
finish this until I get back (20th), but I wanted to send out a
preview of the patch so that others can take a look.

The fundamental issue with that bug is the api mismatch between gold
and our own LTO api. The gold api talks about "symbol foo in file
bar". Our api talks about "symbol foo in the merged module".

In that particular testcase, what happens is that gold asks us to keep
a particular linkonce_odr symbol, but the IR linker doesn't copy it to
the merged module and we never have a chance to ask lib/LTO to keep
it.

The fix, I think, is to have a more direct implementation of the gold
api. If it asks us to keep a symbol, we change the linkage so it is
not linkonce. If it says we can drop a symbol, we do so. All of this
before we even send the module to lib/Linker.

Another advantage is that we can drop some linkonce_odr symbols from
the symbol table without having to read the entire module upfront,
only after symbol resolution is done.

The attached patched is quiet a hack. The final intention is to
refactor lib/LTO to expose the API we need, not to copy and paste code
as the patch does :-)

The patch is also missing some option handling (setting codegen
attributes for example) and lto logic (don't internalize memcpy for
example).

Some interesting properties:

* During symmbol resolution we use a temporary LLVMContext and do lazy
module loading. This allows us to keep the minimum possible amount of
allocated memory around. This should also allow as much parallelism as
we want, since there is no shared context.

* During codegen, we can use a plain Module and trim it before
linking. This will need some small changes to handle non-elf mangling,
but I don't expect it to be too hard.

* Since we know what to do with each global, it should be able to make
the reading parallel, but in here llvmcontext synchronization would be
an issue.

A similar organization is hopefully possible in lld.

I just coded enough for it to bootstrap with lto enabled. Next items on my list:

* Vacations :-)
* Feature parity and LTO refactoring to avoid code duplication. With
this it should already be an strict improvement.
* Add support for COMDATs now that we have them in the IR. This should
allow us to use gcc style comdats for c++ constructors and
destructors.
* try to support bfd ld with COFF, as a testcase where mangling in relevant.
* Try to improve how we represent lazy function bodies to make this
more efficient.
* Figure out a better interface with lib/Linker, since right now it is
"fighting" with the plugin to see who is in charge of symbol
resolution.
* Implement lazy loading of metadata.

Cheers,
Rafael
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