Change the code to no longer compute the "type description" immediately when

the type is analyzed.  Instead, only compute it when requested (with
getDescription), and cached for reuse later.

This dramatically speeds up LLVM in general because these descriptions almost
_never_ need to be constructed.  The only time they are used is when a type is
<<'d.  Printing of modules by themselves uses other code to print symbolic
types when possible, so these descriptions are really only used for debugging.

Also, this fixes the particularly bad case when lots of types get resolved to
each other, such as during linking of large programs.  In these cases, the type
descriptions would be repeatedly recomputed and discarded even though: A. noone
reads the description before it gets resolved, and B. many many resolutions
happen at intermediate steps, causing a HUGE waste of time.

Overall, this makes the getTypeDesc function much more light-weight, and fixes
bug: Assembler/2002-07-08-HugePerformanceProblem.llx, which went from taking
1048.770u/19.150s (which is 17.5 MINUTES, on apoc), to taking 0.020u/0.000s,
which is a nice little speedup.  :)

llvm-svn: 8320