[clang] [llvm] Added instant events and marking defered templated instantiation. (PR #103039)

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

What is the reason for changing from `llvm::TimeTraceProfilerEntry*` to `std::shared_ptr<DurableEvent>`? We are also adding significant devirtualization cost with the added different event types.
Can we do something like this:
```
enum class TimeTraceEventType { DurationEvent, InstantEvent };
struct llvm::TimeTraceProfilerEntry {
  const TimePointType Start;
  TimePointType End;
  const std::string Name;
  TimeTraceMetadata Metadata;
  TimeTraceEventType EventType;
  std::vector<std::unique_ptr<TimeTraceProfilerEntry>> InstantEvents; // Instant event associated to a duration event.
}
```
This will remove the need for different implementations for each event type and reduce the complexity.

We can also move the `TimeTraceProfilerEntry` decl to `TimeProfiler.h`. I do not see a point of having just a forward decl

https://github.com/llvm/llvm-project/pull/103039