[C++] Vendored datetime has unchanged header guards

[K-ballo]

Describe the bug, including details regarding any error messages, version, and platform.

Arrow ships with a vendored date library that places all definitions in the arrow_vendored namespace but keeps the original header guards, leading to problems when used in a project that also consumes the real date library.

Suggested fix: prepend all header guards with ARROW_VENDORED_.

Component(s)

C++

[kou]

Good catch. Do you want to work on this?

We also want to automate it by updating this part:

arrow/cpp/src/arrow/vendored/datetime/update.sh

Lines 45 to 48 in 2719c9c

	sed -i.bak -E \
	-e 's/namespace date/namespace arrow_vendored::date/g' \
	-e 's,include "date/,include ",g' \
	*.{cpp,h,mm}