Completely rewrote the class. SymbolTable now separates Type* from Value* in preparation\

for making Type not derive from Value. There are now separate interfaces \
for looking up, finding, and inserting Types and Values. There are also \
three separate iterator interfaces, one for type planes, one for the types \
(type type plane), and one for values within a type plane. See the \
documentation in the Header file.

llvm-svn: 13752