Changes

[llvm-readef/obj] - Change the design structure of ELF dumper. NFCI.

This is a refactoring for design of stuff in `ELFDumper.cpp`.
The current design of ELF dumper is far from ideal.

Currently most overridden functions (inherited from `ObjDumper`) in `ELFDumper` just forward to
the functions of `ELFDumperStyle` (which can be either `GNUStyle` or `LLVMStyle`).
A concrete implementation may be in any of `ELFDumper`/`DumperStyle`/`GNUStyle`/`LLVMStyle`.

This patch reorganizes the classes by introducing `GNUStyleELFDumper`/`LLVMStyleELFDumper`
which inherit from `ELFDumper`. The implementations are moved:

`DumperStyle` -> `ELFDumper`
`GNUStyle` -> `GNUStyleELFDumper`
`LLVMStyle` -> `LLVMStyleELFDumper`

With that we can avoid having a lot of redirection calls and helper methods.
The number of code lines changes from 7142 to 6922 (reduced by ~3%) and the
code overall looks cleaner.

Differential revision: https://reviews.llvm.org/D93900

[llvm-readobj] - Add 'override' to fix build bots.

This should fix bots after landing D93900.

An example of error is:

/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/tools/llvm-readobj/ELFDumper.cpp:883:8: warning: 'printSectionMapping' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void printSectionMapping() {}

[llvm-readobj] - An attempt to fix BB.

This adds the `template` keyword for 'getAsArrayRef' calls.

An example of error:
/b/1/openmp-gcc-x86_64-linux-debian/llvm.src/llvm/tools/llvm-readobj/ELFDumper.cpp:4491:50: error: use 'template' keyword to treat 'getAsArrayRef' as a dependent template name
    for (const Elf_Rel &Rel : this->DynRelRegion.getAsArrayRef<Elf_Rel>())

[mlir][openmp][NFCI] Rename `continuationIP` to `continuationBlock`

Argument is a `Block` not a `point`.

[llvm-readobj] - One more attempt to fix BB.

Add `this->` for `W`, which is the member of `ObjDumper`

An example of error:
readobj/ELFDumper.cpp:738:13: error: use of undeclared identifier 'W'
    assert(&W.getOStream() == &llvm::fouts());