llvm-dis: Stop crashing when dropping debug info

Since r199356, we've printed a warning when dropping debug info.
r225562 started crashing on that, since it registered a diagnostic
handler that only expected errors.  This fixes the handler to expect
other severities.  As a side effect, it now prints "error: " at the
start of error messages, similar to `llvm-as`.

There was a testcase for r199356, but it only really checked the
assembler.  Move `test/Bitcode/drop-debug-info.ll` to `test/Assembler`,
and introduce `test/Bitcode/drop-debug-info.3.5.ll` (and companion
`.bc`) to test the bitcode reader.

Note: tools/gold/gold-plugin.cpp has an equivalent bug, but I'm not sure
what the best fix is there.  I'll file a PR.

llvm-svn: 230416
diff --git a/llvm/test/Bitcode/invalid.ll b/llvm/test/Bitcode/invalid.ll
index 8b532d8..df9fec8 100644
--- a/llvm/test/Bitcode/invalid.ll
+++ b/llvm/test/Bitcode/invalid.ll
@@ -1,6 +1,6 @@
 ; RUN:  not llvm-dis < %s.bc 2>&1 | FileCheck %s
 
-; CHECK: llvm-dis{{(\.EXE|\.exe)?}}: Unknown attribute kind (48)
+; CHECK: llvm-dis{{(\.EXE|\.exe)?}}: error: Unknown attribute kind (48)
 
 ; invalid.ll.bc has an invalid attribute number.
 ; The test checks that LLVM reports the error and doesn't access freed memory