[asan] Create a .ASAN$G(A-Z) section for global registration

Summary:
The expectation is that new instrumented code will add global variable
metadata to the .ASAN$GL section, and we will use this new code to
iterate over it.

This technique seems to break when using incremental linking, which
seems to align every global to a 256 byte boundary. Presumably this is
so that it can incrementally cope with global changing size. Clang
already passes -incremental:no as a linker flag when you invoke it to do
the link step.

The two tests added for this feature will fail until the LLVM
instrumentation change in D26770 lands, so they are marked XFAIL for
now.

Reviewers: pcc, kcc, mehdi_amini, kubabrecka

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D26771

llvm-svn: 287246