Switch ValueSymbolTable to use StringMap<Value*> instead of std::map<std::string...

Switch ValueSymbolTable to use StringMap<Value*> instead of std::map<std::string, Value*>
as its main datastructure.  There are many improvements yet to be made, but
this speeds up opt --std-compile-opts on 447.dealII by 7.3%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34193 91177308-0d34-0410-b5e6-96231b3b80d8