move getNameWithPrefix and getSymbol to TargetMachine.

TargetLoweringBase is implemented in CodeGen, so before this patch we had
a dependency fom Target to CodeGen. This would show up as a link failure of
llvm-stress when building with -DBUILD_SHARED_LIBS=ON.

This fixes pr18900.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201711 91177308-0d34-0410-b5e6-96231b3b80d8