[PATCH] Avoid unnecessary inclusion of <stdexcept> header

[Jonathan Wakely]

This can greatly reduce the amount of preprocessed code that is included
by other headers, because <stdexcept> depends on <string> which is huge.

	* include/std/array: Do not include <stdexcept>.
	* include/std/optional: Include <exception> and
	<bits/exception_defines.h> instead of <stdexcept>.

Preprocessed line counts for C++17 mode:

       <tuple> <memory> <map>
Before   25774    32453 31616
After     9925    23194 19062

Tested x86_64-linux, committed to trunk.

Once we have a gcc-10/porting_to.html page I'll note this change
there, because code relying on std::string and std::allocator being
defined by transitive includes will need to include the right headers.

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