New formatting symbol %# can be used in summary strings to get the "count of...

New formatting symbol %# can be used in summary strings to get the "count of children" of a variable
 - accordingly, the test cases for the synthetic providers for the std:: containers have been edited to use
   ${svar%#} instead of ${svar.len} to print out the count of elements ; the .len synthetic child has been
   removed from the synthetic providers
The synthetic children providers for the std:: containers now return None when asked for children indexes >= num_children()
Basic code to support filter names based on regular expressions (WIP)

llvm-svn: 136862