Optimization for AppleDescriptions.convertToFlatCxxHeaders
Optimization for AppleDescriptions.convertToFlatCxxHeaders

Summary:
I profiled `buck project` with YourKit.
Rebuilding the `ImmutableSortedMap` using a `HashMap` was expensive.
I replaced it with a different implementation which seems to perform better.

before diff:
```
method                                                                                                        time self
com.facebook.buck.apple.AppleDescriptions.convertToFlatCxxHeaders(Path, Function, Set) AppleDescriptions.java 5636 254
```

after diff:
```
method                                                                                                        time self
com.facebook.buck.apple.AppleDescriptions.convertToFlatCxxHeaders(Path, Function, Set) AppleDescriptions.java 3888 79
```

Test Plan: -

Reviewed By: illicitonion

fbshipit-source-id: 4ae3783

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