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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232885 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Hexagon/HexagonTargetMachine.h

@@ -32,7 +32,7 @@ public:
                        Reloc::Model RM, CodeModel::Model CM,
                        CodeGenOpt::Level OL);
   ~HexagonTargetMachine() override;
-  const HexagonSubtarget *getSubtargetImpl() const override {
+  const HexagonSubtarget *getSubtargetImpl(const Function &) const override {
     return &Subtarget;
   }
   static unsigned getModuleMatchQuality(const Module &M);