--- a/taskcluster/scripts/misc/build-clang-windows-helper32.sh
+++ b/taskcluster/scripts/misc/build-clang-windows-helper32.sh
@@ -16,16 +16,20 @@ fi
 ./build/src/mach artifact toolchain -v --authentication-file="${TOOLTOOL_AUTH_FILE}" --tooltool-manifest "build/src/${TOOLTOOL_MANIFEST}"${TOOLTOOL_CACHE:+ --cache-dir ${TOOLTOOL_CACHE}}${MOZ_TOOLCHAINS:+ ${MOZ_TOOLCHAINS}}
 
 # Set up all the Visual Studio paths.
 MSVC_DIR=vs2015u3
 VSWINPATH="$(cd ${MSVC_DIR} && pwd)"
 
 echo vswinpath ${VSWINPATH}
 
+# LLVM_ENABLE_DIA_SDK is set if the directory "$ENV{VSINSTALLDIR}DIA SDK"
+# exists.
+export VSINSTALLDIR="${VSWINPATH}/"
+
 export WINDOWSSDKDIR="${VSWINPATH}/SDK"
 export WIN32_REDIST_DIR="${VSWINPATH}/VC/redist/x86/Microsoft.VC140.CRT"
 export WIN_UCRT_REDIST_DIR="${VSWINPATH}/SDK/Redist/ucrt/DLLs/x86"
 
 export PATH="${VSWINPATH}/VC/bin/amd64_x86:${VSWINPATH}/VC/bin/amd64:${VSWINPATH}/VC/bin:${VSWINPATH}/SDK/bin/x86:${VSWINPATH}/SDK/bin/x64:${VSWINPATH}/DIA SDK/bin:${PATH}"
 export PATH="${VSWINPATH}/VC/redist/x86/Microsoft.VC140.CRT:${VSWINPATH}/VC/redist/x64/Microsoft.VC140.CRT:${VSWINPATH}/SDK/Redist/ucrt/DLLs/x86:${VSWINPATH}/SDK/Redist/ucrt/DLLs/x64:${PATH}"
 
 export INCLUDE="${VSWINPATH}/VC/include:${VSWINPATH}/VC/atlmfc/include:${VSWINPATH}/SDK/Include/10.0.14393.0/ucrt:${VSWINPATH}/SDK/Include/10.0.14393.0/shared:${VSWINPATH}/SDK/Include/10.0.14393.0/um:${VSWINPATH}/SDK/Include/10.0.14393.0/winrt:${VSWINPATH}/DIA SDK/include"

--- a/taskcluster/scripts/misc/build-clang-windows-helper64.sh
+++ b/taskcluster/scripts/misc/build-clang-windows-helper64.sh
@@ -16,16 +16,20 @@ fi
 ./build/src/mach artifact toolchain -v --authentication-file="${TOOLTOOL_AUTH_FILE}" --tooltool-manifest "build/src/${TOOLTOOL_MANIFEST}"${TOOLTOOL_CACHE:+ --cache-dir ${TOOLTOOL_CACHE}}${MOZ_TOOLCHAINS:+ ${MOZ_TOOLCHAINS}}
 
 # Set up all the Visual Studio paths.
 MSVC_DIR=vs2015u3
 VSWINPATH="$(cd ${MSVC_DIR} && pwd)"
 
 echo vswinpath ${VSWINPATH}
 
+# LLVM_ENABLE_DIA_SDK is set if the directory "$ENV{VSINSTALLDIR}DIA SDK"
+# exists.
+export VSINSTALLDIR="${VSWINPATH}/"
+
 export WINDOWSSDKDIR="${VSWINPATH}/SDK"
 export WIN32_REDIST_DIR="${VSWINPATH}/VC/redist/x64/Microsoft.VC140.CRT"
 export WIN_UCRT_REDIST_DIR="${VSWINPATH}/SDK/Redist/ucrt/DLLs/x64"
 
 export PATH="${VSWINPATH}/VC/bin/amd64:${VSWINPATH}/VC/bin:${VSWINPATH}/SDK/bin/x64:${VSWINPATH}/VC/redist/x64/Microsoft.VC140.CRT:${VSWINPATH}/SDK/Redist/ucrt/DLLs/x64:${VSWINPATH}/DIA SDK/bin/amd64:${PATH}"
 
 export INCLUDE="${VSWINPATH}/VC/include:${VSWINPATH}/VC/atlmfc/include:${VSWINPATH}/SDK/Include/10.0.14393.0/ucrt:${VSWINPATH}/SDK/Include/10.0.14393.0/shared:${VSWINPATH}/SDK/Include/10.0.14393.0/um:${VSWINPATH}/SDK/Include/10.0.14393.0/winrt:${VSWINPATH}/DIA SDK/include"
 export LIB="${VSWINPATH}/VC/lib/amd64:${VSWINPATH}/VC/atlmfc/lib/amd64:${VSWINPATH}/SDK/lib/10.0.14393.0/ucrt/x64:${VSWINPATH}/SDK/lib/10.0.14393.0/um/x64:${VSWINPATH}/DIA SDK/lib/amd64"