[ThinLTO] Optionally ignore empty index file

Summary:
In order to simplify distributed build system integration, where actions
may be scheduled before the Thin Link which determines the list of
objects selected by the linker. The gold plugin currently will emit
0-sized index files for objects not selected by the link, to enable
checking for expected output files by the build system. If the build
system then schedules a backend action for these bitcode files, we want
to be able to fall back to normal compilation instead of failing.

This is the LLVM side support for optionally enabling fallback
instead of issuing an error. Return a null CombinedIndex from
llvm::getModuleSummaryIndexForFile under the option when the file
is empty. Clang can then ignore the index when it is null.

Clang patch is D28362.

Reviewers: mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28410

llvm-svn: 291302