This avoids doing a Triple -> std::string -> Triple round trip in lots
of places, now that the Module stores a Triple.

Previously this took a reference to a map and returned a bool to say
whether it succeeded. We can return a StringMap instead, as all callers
but 1 simply iterated the map if the bool was true, and passed in empty
maps as the starting point.

lldb's lit-cpuid did specifically check whether the call failed, but due
to the way the x86 routines work this works out the same as checking if
the returned map is empty.

This PR exposes four APIs to C/OCaml bindings for `TargetMachine`:
```
/** Enable fast-path instruction selection. */
void LLVMSetTargetMachineFastISel(LLVMTargetMachineRef T, LLVMBool Enable);

/** Enable global instruction selection. */
void LLVMSetTargetMachineGlobalISel(LLVMTargetMachineRef T, LLVMBool Enable);

/** Set abort behaviour when global instruction selection fails to lower/select
 * an instruction. */
void LLVMSetTargetMachineGlobalISelAbort(LLVMTargetMachineRef T,
                                         LLVMGlobalISelAbortMode Mode);

/** Enable the MachineOutliner pass. */
void LLVMSetTargetMachineMachineOutliner(LLVMTargetMachineRef T,
                                         LLVMBool Enable);
```

Fixes #70666.

The ABI parameter is used by a number of common backends, including ARM,
MIPS, PPC, and RISCV. Exposing it via the C API makes it possible for
users of those backends to configure the ABI without custom bindings.

This will make it easy for callers to see issues with and fix up calls
to createTargetMachine after a future change to the params of
TargetMachine.

This matches other nearby enums.

For downstream users, this should be a fairly straightforward
replacement,
e.g. s/CodeGenOpt::Aggressive/CodeGenOptLevel::Aggressive
or s/CGFT_/CodeGenFileType::

SubtargetFeature.h is currently part of MC while it doesn't depend on
anything in MC. Since some LLVM components might have the need to work
with target features without necessarily needing MC, it might be
worthwhile to move SubtargetFeature.h to a different location. This will
reduce the dependencies of said components.

Note that I choose TargetParser as the destination because that's where
Triple lives and SubtargetFeatures feels related to that.

This issues came up during a JITLink review (D149522). JITLink would
like to avoid a dependency on MC while still needing to store target
features.

Reviewed By: MaskRay, arsenm

Differential Revision: https://reviews.llvm.org/D150549

The forwarding header is left in place because of its use in
`polly/lib/External/isl/interface/extract_interface.cc`, but I have
added a GCC warning about the fact it is deprecated, because it is used
in `isl` from where it is included by Polly.