walk exports to distinguish nothrow operators via mangled syms on Windows

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 02, 2013 10:19:25_

We want to replace the operator layer for several reasons (avoid MS
abstraction violations ( issue #26 , issue #500 , issue #832 ) and report mismatch errors),
but we have to distinguish nothrow and especially placement operators.

This case covers using mangled syms on Windows to distinguish nothrow when
we don't have type info. We already do this for Linux.

Today we ignore the nothrow distinction in some cases when we can't
distinguish, arguing it's low-risk:

```
* XXX i#: We'll give up on nothrow: but it's low-risk b/c it only comes into
* play if we run out of memory. Plus -replace_malloc doesn't throw
* an exception yet anyway.
```

xref issue #1006

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1206_