Add 'explicit' to darwin embedder constructors

[jmagman]

Add explicit to constructor declarations in macOS host_debug and ios_debug

Include headers using header filter regex:

Checks: '-*,google-explicit-constructor'
WarningsAsErrors: '-*,google-explicit-constructor'
HeaderFilterRegex: '.*/flutter/.*h$'

https://clang.llvm.org/extra/clang-tidy/checks/google-explicit-constructor.html

macOS and iOS version of #29741
For flutter/flutter#93576

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[zanderso]

I don't know how painful it would be, but I think these two are cases where we would want to have explicit.

[jmagman]

It worked, in this case I was confused about which one was erroring.

[jmagman]

This one should be explicit I think.

[jmagman]

luci-engine status is wrong, infra is investigating. Tree is actually green, merging.