Hoist out from LVDWARFReader and LVBinaryReader some generic
code, so it can be available to other readers that do not share the
binary format.

lookupTarget accepts StringRef.  We don't need to create a temporary
instance of std::string only to be converted back to StringRef.

This pull request fixes a couple of unhandled situations in DWARF input
leading to a crash. Specifically,

- If the DWARF input contains a declaration of a C variadic function
(where `...` translates to `DW_TAG_unspecified_parameters`), which is
then followed by a definition, `llvm_unreachable()` is hit in
`LVScope::addMissingElements()`. This is only visible in Debug builds.

- Parsing of instructions in `LVBinaryReader::createInstructions()` does
not check whether `Offset` lies within the `Bytes` ArrayRef. A specially
crafted DWARF input can lead to this condition.

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

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  53 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".

Add support for the WebAssembly binary format and be able to generate
logical views.

https://github.com/llvm/llvm-project/issues/69181

The README.txt includes information about how to build the test cases.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D150996