"unknown module" in LeakSanitizer traces

I am seeing a lot of these with a program built with Clang 5.0.1 on ubuntu 16.04

```
Direct leak of 32768 byte(s) in 8 object(s) allocated from:
#0 0x14d2a23 in malloc /opt/media/clang_nightly/llvm/utils/release/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:67:3
#1 0x7f46220c3faf ()
```

The compiler flags include the following
```
-fsanitize=address -g -fno-omit-frame-pointer -fno-optimize-sibling-calls
```

I installed libstdc++5-dbgsym but that didn't help.

I also tried ASAN_OPTIONS="fast_unwind_on_malloc=0 malloc_context_size=2" but that didn't help with these leaks and even showed less information for other leaks which are giving proper backtraces. Using it without "malloc_context_size" doesn't seem to do anything - the application isn't starting and the machine is almost completely 100% idle.

As suggested in the FAQ I added "-shared-libstdc++", but that doesn't seem to be available for Clang - it's not even listed in the list of support command-line options.

Reproduce the trace with Clang 5.0.1 on Ubuntu 16.04 using the reported sanitizer flags and ASAN_OPTIONS variants. Start with the symbolization path referenced at asan_malloc_linux.cc:67:3 and determine why the allocation frame is shown as an unknown module; done means an identified cause and a documented or testable resolution.

Written by the indexing model from the issue text.