Changes

clang-ve-staging: Staging builder for VE work-in-progress features

[clang-tidy][docs][NFC] Improve documentation of bugprone-unhandled-exception-at-new

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D114602

[libc] Select FPUtils implementations via code instead of build

We want to simplify the build system and rely on code to do the implementation selection.
This is in preparation of adding a Bazel configuration (D114712).

Differential Revision: https://reviews.llvm.org/D115034

[MemoryLocation] Move DSE intrinsic handling to MemoryLocation. (NFC)

Suggested in D114872.

[gn build] Build with Fission on non-mac non-win when using lld

In release+sym builds (-O2 -g), reduces time to link `clang`
from 2.3s to 1.3s (-42%).

In debug builds (-g), reduces time to link `clang`
from 5.4s to 4.5s (-17.4%).

See the phab review for full `ministat` numbers.

In the CMake build this is opt-in via LLVM_USE_SPLIT_DWARF.
Since the GN build is targeted at developers, enabling it by default
seems like a better default setting here. (If it turns out to cause
problems, we can add an opt-out.)

Time to load the binary into gdb and to set a breakpoint is unchanged.
Time from `run` to hitting a breakpoint in `main` feel a bit faster
(~4s -> ~2s), but I dind't do a careful statistical anlysis for this.

Differential Revision: https://reviews.llvm.org/D115040

[libc] Fix invalid include for SqrtLongDouble.h

[ARM] Replace if's with a switch, NFC

I'm not having a lot of luck with the microosft compiler recently. Maybe
this will help it with its errors:
llvm\lib\IR\AutoUpgrade.cpp(3726): fatal error C1061: compiler limit: blocks nested too deeply

If not, it's a good code cleanup anyway.