Try to use DiagnosticInfo if every register in the class is reserved
by forcing assignment to a reserved register. Also reduces the number
of redundant errors emitted, particularly with fast.

This is still broken in the case of undef uses. There are additional
complications in greedy and fast, so leave it for a separate fix.

Following discussions in #110443, and the following earlier discussions
in https://lists.llvm.org/pipermail/llvm-dev/2017-October/117907.html,
https://reviews.llvm.org/D38482, https://reviews.llvm.org/D38489, this
PR attempts to overhaul the `TargetMachine` and `LLVMTargetMachine`
interface classes. More specifically:
1. Makes `TargetMachine` the only class implemented under
`TargetMachine.h` in the `Target` library.
2. `TargetMachine` contains target-specific interface functions that
relate to IR/CodeGen/MC constructs, whereas before (at least on paper)
it was supposed to have only IR/MC constructs. Any Target that doesn't
want to use the independent code generator simply does not implement
them, and returns either `false` or `nullptr`.
3. Renames `LLVMTargetMachine` to `CodeGenCommonTMImpl`. This renaming
aims to make the purpose of `LLVMTargetMachine` clearer. Its interface
was moved under the CodeGen library, to further emphasis its usage in
Targets that use CodeGen directly.
4. Makes `TargetMachine` the only interface used across LLVM and its
projects. With these changes, `CodeGenCommonTMImpl` is simply a set of
shared function implementations of `TargetMachine`, and CodeGen users
don't need to static cast to `LLVMTargetMachine` every time they need a
CodeGen-specific feature of the `TargetMachine`.
5. More importantly, does not change any requirements regarding library
linking.

cc @arsenm @aeubanks

Identified with misc-include-cleaner.

A previous commit d826b0c9 accidentally added a new MachineFunctionProperty,
HasFakeUses, that was unused by the commit (and results in an
uncovered-switch warning, which was fixed by a separate followup 1811e872);
this patch removes that enum value.

llvm-project/llvm/lib/CodeGen/MachineFunction.cpp:95:10:
error: enumeration value 'HasFakeUses' not handled in switch [-Werror,-Wswitch]
  switch(Prop) {
         ^~~~
1 error generated.

This reapplies commit 1911a50fae8a441b445eb835b98950710d28fc88 with a
minor fix in lld/ELF/LTO.cpp which sets Options.BBAddrMap when
`--lto-basic-block-sections=labels` is passed.

This reverts commit 1911a50fae8a441b445eb835b98950710d28fc88.

Several bots are failing:

https://lab.llvm.org/buildbot/#/builders/190/builds/6519
https://lab.llvm.org/buildbot/#/builders/3/builds/5248
https://lab.llvm.org/buildbot/#/builders/18/builds/4463

This feature is supported via the newer option
`-fbasic-block-address-map`. Using the old option still works by
delegating to the newer option, while a warning is printed to show
deprecation.

The dominator tree gained an optimization to use block numbers instead
of a DenseMap to store blocks. Given that machine basic blocks already
have numbers, expose these via appropriate GraphTraits. For debugging,
block number epochs are added to MachineFunction -- this greatly helps
in finding uses of block numbers after RenumberBlocks().

In a few cases where dominator trees are preserved across renumberings,
the dominator tree is updated to use the new numbers.