Add facilities for extract-api:
- Structs/classes to hold collected API information: `APIRecord`, `API`
- Structs/classes for API information:
- `AvailabilityInfo`: aggregated availbility information
- `DeclarationFragments`: declaration fragments
- `DeclarationFragmentsBuilder`: helper class to build declaration
fragments for various types/declarations
- `FunctionSignature`: function signature
- Serialization: `Serializer`
- Add output file for `ExtractAPIAction`
- Refactor `clang::RawComment::getFormattedText` to provide an
additional `getFormattedLines` for a more detailed view of comment lines
used for the SymbolGraph format
Add support for global records (global variables and functions)
- Add `GlobalRecord` based on `APIRecord` to store global records'
information
- Implement `VisitVarDecl` and `VisitFunctionDecl` in `ExtractAPIVisitor` to
collect information
- Implement serialization for global records
- Add test case for global records
Differential Revision: https://reviews.llvm.org/D119479
