llvm@4: fix C++ compilation for Mojave and Catalina

[smorimoto]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

Port from #45693. Thank you @jamiesnape.

Related issue and PR: #45061 and #45304.

[smorimoto]

@BrewTestBot test this please!

[fxcoudert]

/tmp/llvm@4-20191030-5941-10nafpo/llvm-4.0.1.src/tools/lld/ELF/LTO.cpp:161:32: error: 'template' keyword not permitted here
template void BitcodeCompiler::template add<ELF32LE>(BitcodeFile &);
                               ^~~~~~~~~
/tmp/llvm@4-20191030-5941-10nafpo/llvm-4.0.1.src/tools/lld/ELF/LTO.cpp:162:32: error: 'template' keyword not permitted here
template void BitcodeCompiler::template add<ELF32BE>(BitcodeFile &);
                               ^~~~~~~~~
/tmp/llvm@4-20191030-5941-10nafpo/llvm-4.0.1.src/tools/lld/ELF/LTO.cpp:163:32: error: 'template' keyword not permitted here
template void BitcodeCompiler::template add<ELF64LE>(BitcodeFile &);
                               ^~~~~~~~~
/tmp/llvm@4-20191030-5941-10nafpo/llvm-4.0.1.src/tools/lld/ELF/LTO.cpp:164:32: error: 'template' keyword not permitted here
template void BitcodeCompiler::template add<ELF64BE>(BitcodeFile &);
                               ^~~~~~~~~

Possibly this is just too old to compile with Xcode 11

[smorimoto]

This should work.

template void BitcodeCompiler::add<ELF32BE>(BitcodeFile &);

[smorimoto]

Oh, it's the LLVM source code... Possibly it can't build on Mojave or later. I guess.

[fxcoudert]

Given it's an old version, we'll keep it as is. Thanks @imbsky nonetheless for this PR, and all the others!

[smorimoto]

@fxcoudert I think that's fine. Nooo! Review, merge, and manage are really hard work than most people think! Thank you!