Deprecate FatalWarnings flags in favor of fatalwarnings API

[nickclark2016]

What does this PR do?

Moves the existing FatalWarnings, FatalCompileWarnings, and FatalLinkWarnings flags to the existing fatalwarnings API.

How does this PR change Premake's behavior?

Deprecates the FatalWarnings (and friends) flags. Translates the calls to:

flags { "FatalWarnings" }
-- becomes
fatalwarnings { "Compile", "Link" }

flags { "FatalCompileWarnings" }
-- becomes
fatalwarnings { "Compile" }

flags { "FatalLinkWarnings" }
-- becomes
fatalwarnings { "Link" }

Anything else we should know?

Deprecation of flags ahead of 5.0 release.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

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[Jarod42]

Testing filterFatalWarnings might make sense, i.e:

fatalwarnings { "Link", "4996" }

[samsinsane]

I can see that src/tools/clang.lua hasn't been modified, I assume it needs to be updated?

[samsinsane]

For a future PR, we should consider an All value for those that used FatalWarnings before.