[LFI] Add MCLFIRewriter infrastructure (#172906)

This is the second patch in the LFI series, adding the following
features:

* `MCLFIRewriter` class, which will be used to perform LFI rewrites on a
per-architecture basis. Each architecture where LFI is supported will
implement a subclass (for example,
`Target/AArch64/MCTargetDesc/AArch64MCLFIRewriter.cpp`) that will
implement the `rewriteInst` function to perform the actual rewriting
(the AArch64 version will be added in the next PR). The generic rewriter
class provides some instruction info utilities (`mayLoad`, `mayStore`)
and is used to call `rewriteInst` during instruction emission. It also
provides `onLabel` which allows the rewriter to know possible branch
targets, making certain optimizations like guard elimination possible.
* LFI streamer initialization that marks object files with a NOTE
section to indicate that the object file is using LFI.
* A basic LFI assembly parser that introduces the `.lfi_rewrite_disable`
and `.lfi_rewrite_enable` directives that can be used to control whether
rewriting is enabled or not in hand-written assembly.