Add the "block" keyword to "thread step-in -e", and an alias that uses it:...

Add the "block" keyword to "thread step-in -e", and an alias that uses it: "sif <target function>" - i.e. step-into-function
to allow you to step through a complex calling sequence into a particular function that may span multiple lines.  Also some
test cases for this and the --step-target feature.

llvm-svn: 261953