[TableGen] Replace InfoByHwMode::getAsString with writeToStream

[TableGen] Replace InfoByHwMode::getAsString with writeToStream

Also add operator<< for use with raw_ostream to InfoByHwMode and its
derived classes.

Recommitting r313989 with the fix for unresolved references: explicitly
define the operator<< in namespace llvm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314004 91177308-0d34-0410-b5e6-96231b3b80d8