Remove the isImplicitlyPrivate argument of getNameWithPrefix.

getSymbolWithGlobalValueBase use is to create a name of a new symbol based
on the name of an existing GV. Assert that and then remove the last call
to pass true to isImplicitlyPrivate.

This gives the mangler API a 1:1 mapping from GV to names, which is what we
need to drop the mangler dependency on the target (and use an extended
datalayout instead).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196472 91177308-0d34-0410-b5e6-96231b3b80d8