Auto merge of #75908 - pietroalbini:ci-bump-llvm, r=Mark-Simulacrum

Bump LLVM on CI to 10.0.0

This PR bumps the LLVM version we use on our macOS and Windows CI to 10.0.0, fixing a breakage we noticed today:

```
2020-08-25T14:24:28.5939568Z FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/AMDGPUMetadata.cpp.obj
2020-08-25T14:24:28.5940317Z D:\a\rust\rust\build\bootstrap\debug\sccache-plus-cl.exe  /nologo -TP -DGTEST_HAS_RTTI=0 -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Ilib\Support -ID:\a\rust\rust\src\llvm-project\llvm\lib\Support -Iinclude -ID:\a\rust\rust\src\llvm-project\llvm\include -nologo -MT -Brepro --target=x86_64-pc-windows-msvc /Zc:inline /Zc:strictStrings /Oi /Zc:rvalueCast /Brepro /W4  -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wno-noexcept-type -Wno-comment /Gw /MT /O2 /Ob2 -UNDEBUG -std:c++14  /EHs-c- /GR- /showIncludes /Folib\Support\CMakeFiles\LLVMSupport.dir\AMDGPUMetadata.cpp.obj /Fdlib\Support\CMakeFiles\LLVMSupport.dir\LLVMSupport.pdb -c D:\a\rust\rust\src\llvm-project\llvm\lib\Support\AMDGPUMetadata.cpp
2020-08-25T14:24:28.5940861Z clang-cl: warning: argument unused during compilation: '-Brepro' [-Wunused-command-line-argument]
2020-08-25T14:24:28.5941076Z clang-cl: warning: argument unused during compilation: '-Brepro' [-Wunused-command-line-argument]
2020-08-25T14:24:28.5941321Z In file included from D:\a\rust\rust\src\llvm-project\llvm\lib\Support\AMDGPUMetadata.cpp:15:
2020-08-25T14:24:28.5941545Z In file included from D:\a\rust\rust\src\llvm-project\llvm\include\llvm/ADT/Twine.h:12:
2020-08-25T14:24:28.5941774Z In file included from D:\a\rust\rust\src\llvm-project\llvm\include\llvm/ADT/SmallVector.h:16:
2020-08-25T14:24:28.5942016Z In file included from D:\a\rust\rust\src\llvm-project\llvm\include\llvm/ADT/iterator_range.h:21:
2020-08-25T14:24:28.5942257Z In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\include\iterator:9:
2020-08-25T14:24:28.5942542Z C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\include\yvals_core.h(494,2): error: STL1000: Unexpected compiler version, expected Clang 10.0.0 or newer.
```

I uploaded both the new tarballs to our mirrors bucket.

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