Add 'explicit' to header files

[zanderso]

This PR adds explicit to constructor declarations that are asked for in a host_debug build on Linux using the following .clang-tidy file:

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

For flutter/flutter#93576

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[flutter-dashboard]

This pull request was opened against a branch other than master. Since Flutter pull requests should not normally be opened against branches other than master, I have changed the base to master. If this was intended, you may modify the base back to main. See the Release Process for information about how other branches get updated.

Reviewers: Use caution before merging pull requests to branches other than master, unless this is an intentional hotfix/cherrypick.

[zanderso]

@CaseyHillers why is this PR still blocked?

[CaseyHillers]

@zanderso there's a tree failure now. The status is reporting correctly.

[zanderso]

@CaseyHillers GitHub is telling me that merging is blocked. I should be able to land an lgtm'd PR even when the tree is red. (This one doesn't need to, but I should be able to.)

In particular it says "The base branch restricts merging to authorized users."

[jmagman]

@CaseyHillers changed the base to main, but it still says the base is master...?

[jmagman]

@CaseyHillers changed the base to main, but it still says the base is master...?

Switching back to main unblocked it again. Not sure how it got back to being based on master.