build: harmonize Clang checks and update V8 warning cflags

[targos]

• Set the clang variable in config.gypi so it depends on compiler checks made by the configure script.
• Replace gyp conditions with llvm_version and "0.0" with conditions that use the clang variable.
• Always use clang==1 or clang==0 in gyp conditions

This will help for https://github.com/nodejs/node/pull/52870

[nodejs-github-bot]

Review requested:

• [ ] @nodejs/gyp
• [ ] @nodejs/security-wg
• [ ] @nodejs/v8-update

[targos]

Somehow it broke test-asan.

[targos]

Trying to remove as many -W flags as possible from the V8 build config. I'll re-add them if necessary.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/59013/

[nodejs-github-bot]

Landed in 4338a2839131c9a62648f0320cbfe531068018c2...2b657ccfb3fdf861ad029e0358ba1f02dad07009