[release/3.1] Fix build with clang 10

[omajid]

This fixes 3 different sets of build issues that are seen when compiling with clang 10.

• Clang 10 fails to compile slist.h because the code contained is actually invalid. The assignment operator being used doesn't exist.

  This is a backport of SList::Init: add missing constructor runtime#33096

• Clang 10 has moved exception-handling mismatches in function declarations under the -fms-compatibility flag (instead of the -fms-extensions flag). Our declarations of atoll and other similar functions are missing the exception declaration throw(). This mismatch in exception declarations makes clang 10 unable to build this code. Fix it by defining THROW_DECL as throw() which is supported at least as far back as clang 3.3.

  This is a backport of Fix re-declarations of builtin functions with clang 10 runtime#32837

• Clang 10 has enabled additional warnings. Lets turn of -Werror globally in this release branch by making the -ignorewarnings switch to ./build.sh be the default.

[omajid]

cc @jkotas @janvorli @danmosemsft. Can someone please review this?

[jkotas]

Would it be better to fix most of these problems once and for all by disabling all warnings by default (for release branches)?

[omajid]

Would it be better to fix most of these problems once and for all by disabling all warnings by default (for release branches)?

I think so. I don't know if it would have helped with the first issue which is actually invalid code.

I generally defer to https://flameeyes.blog/2009/02/25/future-proof-your-code-dont-use-werror/ which says:

So please, don’t use -Werror in released code, make it optional, use it during development, but not in released code.

[jkotas]

@omajid Could you please amend this PR with a change to disable werror, and undo changes that will be no longer necessary after that?

[omajid]

@jkotas Done. Turns out only misleading-identation issue was a warning.

As for THROW_DECL, I could, alternatively, pass -fms-compatibility along with -fms-extensions instead of changing the definition of THROW_DECL.

[jkotas]

Thanks!