header-checker: Rename to IRReader

This commit renames `TextFormatToIRReader` to `IRReader`,
`ProtobufTextFormatToIRReader` to `ProtobufIRReader`, and
`JsonToIRReader` to `JsonIRReader`.

Test: ./tests/test.py
Test: ${ANDROID_HOST_OUT}/nativetest64/\
      header-checker-unittests/header-checker-unittests
Change-Id: I42e51eca31f6949f9e74b2e1c711075de22220fe