[ELF] Make overlapping output sections an error

Summary:
While trying to make a linker script behave the same way with lld as it did
with bfd, I discovered that lld currently doesn't diagnose overlapping
output sections. I was getting very strange runtime failures which I
tracked down to overlapping sections in the resulting binary. When linking
with ld.bfd overlapping output sections are an error unless
--noinhibit-exec is passed and I believe lld should behave the same way
here to avoid surprising crashes at runtime.

The patch also uncovered an errors in the tests: arm-thumb-interwork-thunk
was creating a binary where .got.plt was placed at an address overlapping
with .got.

Reviewers: ruiu, grimar, rafael

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D41046

llvm-svn: 323856