[LTO] Allow pass remarks with hotness to be set when emitting to stderr

Summary:
Set setDiagnosticsHotnessRequested before the early exit check for a
diagnostic output file, so that pass remarks with hotness works when
emitting pass remarks to stderr (e.g. via -pass-remarks=.).

Also fix the llvm-lto2 diagnistic handler so that it only calls exit(1)
when the diagnistic is an error type. Otherwise the new test invocation
of llvm-lto2 with -pass-remarks causes it to fail. The new code is
consistent with the diagnostic handler elsewhere (e.g. on the
LLVMContext).

Reviewers: pcc, davide

Subscribers: fhahn, mehdi_amini, llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D46387

llvm-svn: 331569