llvm dev - Dec 2018 - Is .bc extension required in LLVM bitcode file?

When building Boost using LLVM, do I have to somehow have it compile library
files that end with a .bc extension, or is .lib extension fine?  I need someone
familiar with Emscripten to answer this, since I want to do it for Emscripten.
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To add: I changed the project-config.jam file in my Boost source tree to have
Boost.Build use LLVM, but then when I use "bootstrap" command, I see
that it still uses MSVC.  When I looked at the project-config.jam file again at
that point, I saw that its content had been reverted back to the default one for
building with MSVC.  What's going on?  What do I need to do?  Is there an
argument I need to provide to the bootstrap command here so that it'll keep
the configuration file content that I want it to?
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