Downgrade implicit int and implicit function declaration to warning only

The changes in Clang 15.0.0 which enabled these diagnostics as a
warning which defaulted to an error caused disruption for people
working on distributions such as Gentoo. There was an explicit request
to downgrade these to be warning-only in Clang 15.0.1 with the
expectation that Clang 16 will default the diagnostics to an error.

See
https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
for more details on the discussion.

See https://reviews.llvm.org/D133800 for the public review of these
changes.