Expand definition deprecation warning to include constexpr statements.

Clang currently warns on definitions downgraded to declarations
with a const modifier, but not for a constexpr modifier. This patch
updates the warning logic to warn on both inputs, and adds a test to
check the additional case as well.

See also: https://bugs.chromium.org/p/chromium/issues/detail?id=1284718

Differential Revision: https://reviews.llvm.org/D126664