Stop manually downloading hardcoded version of llvm/clang. #537

Job	Run time
Build nuget package with MSVC	2m 18s
Build natvis (x86, Release, Component)	1m 32s
MSVC: Build (x86, Debug)	1m 19s
Build natvis (x86, Release, Standalone)	2m 53s
cross: Cross-build from Linux (i686)	54s
MSVC: Build (x86, Release)	1m 18s
Build natvis (x64, Release, Component)	1m 59s
MSVC: Build (x64, Debug)	1m 18s
Build natvis (x64, Release, Standalone)	2m 13s
MSVC: Build (x64, Release)	1m 18s
Build natvis (arm64, Release, Component)	2m 29s
cross: Cross-build from Linux (x86_64)	52s
MSVC: Build (arm64, Release)	1m 19s
Build natvis (arm64, Release, Standalone)	2m 14s
clang-cl: Build (x86, Debug)	1m 19s
clang-cl: Build (x64, Debug)	1m 15s
Build nuget test (${{ matrix.arch }})	0s
${{ matrix.compiler }}: Test [${{ matrix.test_exe }}] (${{ matrix.arch }}, ${{ matrix.config }})	0s
	26m 30s

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