Sprinkle some PrettyStackEntry magic into the passmanager. With this, we now

get nice and happy stack traces when we crash in an optimizer or codegen.  For
example, an abort put in UnswitchLoops now looks like this:

Stack dump:
0.	Program arguments: clang pr3399.c -S -O3 
1.	<eof> parser at end of file
2.	per-module optimization passes
3.	Running pass 'CallGraph Pass Manager' on module 'pr3399.c'.
4.	Running pass 'Loop Pass Manager' on function '@foo'
5.	Running pass 'Unswitch loops' on basic block '%for.inc'
Abort

llvm-svn: 66260