rename PrettyStackTraceDecl -> PrettyStackTraceActionsDecl.

Introduce a new PrettyStackTraceDecl.
Use it to add the top level LLVM IR generation stuff in 
Backend.cpp to stack traces.  We now get crashes like:

Stack dump:
0.	Program arguments: clang t.c -emit-llvm 
1.	<eof> parser at end of file
2.	t.c:1:5: LLVM IR generation of declaration 'a'
Abort

for IR generation crashes.

llvm-svn: 66153