When lldb stops in a stack frame where we have source level information (file, line number), don't
print the disassembly context around $pc -- just print the filename and line number, even if we 
can't show the source code.  Previously if the source file was not available, lldb would print
the source filename & line number and assembly.  
<rdar://problem/13072951> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180706 91177308-0d34-0410-b5e6-96231b3b80d8