Discussion on the committee reflectors referred me to N2634, the design 
paper for the new SFINAE rules, which says that exceeding an 
implementation limit is a hard error, not a substitution failure.  So 
this patch does away with the tracking of indivitual substitutions and 
just uses push_tinst_level to handle nesting.  On enum11.C the compiler 
crashes before hitting the limit of 1024 due to excessive recursion, so 
I've lowered the default for -ftemplate-depth to 900.

Tested x86_64-pc-linux-gnu, applied to trunk.