[clang][extract-api] Add global record support

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