[clang-tidy] Introduce the WarnIntoHeaders option to modernize-deprecated-headers

Unfortunately, we must restrict the checker to warn for deprecated headers
only if the header is included directly from a c++ source file.

For header files, we cannot know if the project has a C source file
that also directly/indirectly includes the offending header file
otherwise. Thus, it's better to be on the safe side and suppress those
reports.

One can opt-in the old behavior, emitting diagnostics into header files,
if one explicitly sets the WarnIntoHeaders=true, in which case nothing
will be changed.

Reviewed By: LegalizeAdulthood

Differential Revision: https://reviews.llvm.org/D125769