Remove the target independent TargetMachine::getSubtarget and

TargetMachine::getSubtargetImpl routines.

This keeps the target independent code free of bare subtarget
calls while the remainder of the backends are migrated, or not
if they don't wish to support per-function subtargets as would
be needed for function multiversioning or LTO of disparate
cpu subarchitecture types, e.g.

clang -msse4.2 -c foo.c -emit-llvm -o foo.bc
clang -c bar.c -emit-llvm -o bar.bc
llvm-link foo.bc bar.bc -o baz.bc
llc baz.bc

and get appropriate code for what the command lines requested.

llvm-svn: 232885