[clang] allow -fthinlto-index= without -x ir

Summary:
Previously, passing -fthinlto-index= to clang required that bitcode
files be explicitly marked by -x ir. This change makes us detect files
with object file extensions as bitcode files when -fthinlto-index= is
present, so that explicitly marking them is no longer necessary.
Explicitly specifying -x ir is still accepted and continues to be part
of the test case to ensure we continue to support it.

Reviewers: tejohnson, rnk, pcc

Subscribers: mehdi_amini, steven_wu, dexonsmith, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64610

llvm-svn: 366127