compiletest: Migrate TestProps directive handling to a system of named handlers

[Zalathar]

One of the very silly things about directive processing in compiletest is that for each directive in the test file, we proceed to check it against dozens of different directive names in linear sequence, without any kind of indexed lookup, and without any early-exit after a known directive name is found (unless a panic occurs).

This PR is a big step away from that, by taking the iter_directives loop in TestProps::load_from and making all of its directive processing dispatch to a hashtable of individual name-specific handlers instead.

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The handler system is set up in a way that should allow us to add capabilities or change the implementation as needed, without having to mass-modify the existing handlers (e.g. this is why the handler and multi_handler functions are used).

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This PR is focused on mass-migrating all of the TestProps directive processing into handlers. Most of the resulting handlers could obviously be simplified further (e.g. by avoiding the redundant name checks that were needed in the pre-migration code), but I've avoided doing any such simplifications in this PR to keep its scope limited and make reviewing easier.

The patches in this PR have been arranged so that the main migration can be inspected with git diff --color-moved --color-moved-ws=ignore-all-space to verify that it moves all of the relevant lines intact, without modifying or discarding any of them.

r? jieyouxu

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

[Zalathar]

Running a bunch of try jobs to check for unforeseen problems:

@bors try jobs=x86_64-msvc-1,i686-msvc-1,x86_64-mingw-1,test-various,armhf-gnu,aarch64-apple,dist-i586-gnu-i586-i686-musl

[rust-bors]

☀️ Try build successful (CI)
Build commit: dd79bb4 (dd79bb4ea51e0e37cea8fc6fdaef3a0586643b34, parent: 37ec98f5d33c0876a9ffa5ee17dc0895659efe37)