Follow-up to https://reviews.llvm.org/D46527

Fixes #157449

ErrorAsOutParameter's Error* constructor supports cases where an Error might not
be passed in (because in the calling context it's known that this call won't
fail). Most clients always have an Error present however, and for them an Error&
overload is more convenient.

Identified with misc-include-cleaner.

There isn’t really a reason for it not to be a `const&` (afaict), and it
is a bit annoying because some APIs (e.g. `TargetMachine::getTargetTriple()`) 
return a `const Triple&`.

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  70 under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

Detect COFF files by default and allow specifying it with --format
argument.

This is important for ARM64EC, which uses a separated symbol map for EC
symbols. Since K_COFF is mostly compatible with K_GNU, this shouldn't
really make a difference for other targets.

This originally landed as #82642, but was reverted due to test failures
in tests using no symbol table. Since COFF symbol can't express it,
fallback to GNU format in that case.

Reverts llvm/llvm-project#82642 for
lld/test/ELF/invalid/Output/data-encoding.test.tmp.a failures on
Windows.

Detect COFF files by default and allow specifying it with --format
argument.

This is important for ARM64EC, which uses a separated symbol map for EC
symbols. Since K_COFF is mostly compatible with K_GNU, this shouldn't
really make a difference for other targets.