- try_emplace(Key) is shorter than insert({Key, nullptr}).
- try_emplace performs value initialization without value parameters.
- We overwrite values on successful insertion anyway.

try_emplace value-initializes values, so we do not need to pass
nullptr to try_emplace when the value types are raw pointers or
std::unique_ptr<T>.

* Include whether functions are inlinable as they impact whether to add
them into the tbd file and for future verification.
* Fix how clang arguments got passed along, previously spacing was
passed along to CC1 causing search path inputs to look non-existent.

Create a helper method for this operation, so it can be reused in
multiple places.
Additonally move FileType enum into its own header to avoid include
cycles.

ObjCInterfaceRecords roughly align to the objc-classes key in tbd-files.
They condensely represent up to 3 symbols. The problem here is that when
represented this way, we lose granularity when these symbols could have
different linkages or outright don't exist. This can happen frequently
in interoptable code generated by the swift compiler. This adds fields
and utility functions to express unique properties for these symbols. If
the record does represent the same properties across all of its symbols,
it will be treated the same in the TBD. Otherwise it will be printed in
global's section.

Reviewed seperately before by Juergen Ributzka

A distinction that doesn't _usually_ matter is that the
MachO::SymbolKind is really a mapping of entries in TBD files not
symbols. To better understand this, rename the enum so it represents an
encoding mapped to TBDs as opposed to symbols alone.

For example, it can be a bit confusing that "GlobalSymbol" is a enum
value when all of those values can represent a GlobalSymbol.

Introduce RecordVisitor. This is used for different clients that want to
extract information out of RecordSlice types.
The first and immediate use case is for serializing symbol information
into TBD files.