Merge changes Ibc7a9a94,I4e8a0683,I4f741262,Ib19c9666,Ib55dc8bd, ...

* changes:
  header-checker: header-abi-dumper should use ModuleIR instead
  header-checker: Extract common code to ModuleIR
  header-checker: Remove getters from IRReader
  header-checker: Fix minor style problem
  header-checker: Do not expose format-specific details
  header-checker: Rename to IRReader
  header-checker: Split IRDumper and IRReader
  header-checker: Extract ModuleIR
  header-checker: Cleanup namespaces
  header-checker: Restructure the source tree