[llvm] [Instrumentation] Mark instrumented calls as implicit (PR #106447)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

This LGTM with a couple of nits;  it's not clear to me whether using line-zero on coverage-instrumentation calls breaks some convention or developer expectation, as coverage-instrumentation isn't an area I'm familiar with.

https://github.com/llvm/llvm-project/pull/106447