Per their documentation, the `llvm::Initialize*` functions are only
supposed to be called by the main program, not by a library like
toolchain/codegen. Fixes a hang due to a data race in multithreaded
autoupdate.
Add a utility class `Carbon::InitLLVM` to do the common LLVM
initialization shared by all Carbon tools, optionally including
initializing the LLVM targets. Because the LLVM targets add a lot of
binary size, only initialize them for binaries that opt in by depending
on a new target `//common:all_llvm_targets`.
Also fix `//explorer:file_test` and `//explorer:file_test.trace` to
share a binary rather than linking an identical binary twice.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Richard SmithandJon Ross-Perkins