Prefer atos to llvm-symbolizer on Darwin

atos is the default symbolizer on Apple's compiler for quite a few years now.
llvm-symbolizer is quite fragile on Darwin: for example, unless a .dSYM
file was explicitly generated symbolication would not work.
It is also very convenient when the behavior of LLVM open source
compiler matches to that of Apple's compiler on Apple's platform.
Furthermore, llvm-symbolizer is not installed on Apple's platform by
default, which leads to strange behavior during debugging: the test
might fail under lit (where it has llvm-symbolizer) but would run
properly when launched on the command line (where it does not, and atos
would be used).

Indeed, there's a downside: atos does not work properly with inlined
functions, hence the test change.
We do not think that this is a major problem, as users would often
compile with -O0 when debugging, and in any case it is preferable to
symbolizer not being able to symbolize.

Differential Revision: https://reviews.llvm.org/D35745

llvm-svn: 308908