getSectionIDNum may return same value for two different MBBSectionID.
e.g. A Cold type MBBSectionID with number 0 and a Default type
MBBSectionID with number 2 get same value 2 from getSectionIDNum. This
may lead to overwrite of MBBSectionRanges.  Using MBBSectionID itself
as DenseMap key is better choice.

Identified with misc-include-cleaner.

credits to @jmagee

This change removes the `tidyLandingPads` function, which previously
had a few responsibilities:

1. Dealing with the deletion of an invoke, after MachineFunction lowering.
2. Dealing with the deletion of a landing pad BB, after MachineFunction lowering.
3. Cleaning up the type-id list generated by `MachineFunction::addLandingPad`.

Case 3 has been fixed in the generator, and the others are now handled
during table emission.

This change also removes `MachineFunction`'s `addCatchTypeInfo`,
`addFilterTypeInfo`, and `addCleanup` helper fns, as they had a single
caller, and being outlined didn't make it simpler.

Finally, as calling `tidyLandingPads` was effectively the only thing
`DwarfCFIExceptionBase` did, that class has been eliminated.

When no landing pads exist for a function, `@LPStart` is undefined and must be omitted.

EH table is generally not emitted for functions without landing pads, except when the personality function is uknown (`!isNoOpWithoutInvoke(classifyEHPersonality(Per))`). In that case, we must omit `@LPStart` even when machine function splitting is enabled.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D131626

Add SwitchSection to return switchSection. The API will be removed soon.

Follow-up to c031378ce01b8485ba0ef486654bc9393c4ac024 .
The class is mostly consistent now.

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121332