clang update.py: When running tests on Windows, make sure DIA works.
clang update.py: When running tests on Windows, make sure DIA works.

LLVM's DIA reading code either needs DIA registered in the registry,
or it needs msdia140.dll somewhere.  Copy msdia140.dll from the hermetic
toolchain into the bin/ directory so it's next to llvm-symbolizer.exe
and friends.  (This means the tests that package.py runs will only all
pass for people and bots that have access to the hermetic toolchain.)

Also do this after downloading clang.  (Here it's not guaranteed that
people have access to the hermetic toolchain -- the script will fall
back to a MSVC install and copy from there.  Since this
requires msdia140.dll, this means we now require either the hermetic
toolchain or an installed MSVS2015. If this caused problems, we can
reconsider.)

BUG=596201

Review URL: https://codereview.chromium.org/1879753002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386745}

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