Speed up Azure CI installing Windows dependencies

There is known issue where PowerShell is unreasonably slow downloading
files due to an issue with rendering the progress bar, see:

PowerShell/PowerShell#2138

That issue is fixed in PowerShell Core (available in Azure Pipelines as
pwsh.exe) but it can also be worked around by setting:

    $ProgressPreference = 'SilentlyContinue'

Author: lzybkr

.azure-pipelines/steps/install-clang.yml

@@ -27,7 +27,7 @@ steps:
 # Original downloaded here came from
 # http://releases.llvm.org/7.0.0/LLVM-7.0.0-win64.exe
 - script: |
-    powershell -Command "iwr -outf %TEMP%\LLVM-7.0.0-win64.exe https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/LLVM-7.0.0-win64.exe"
+    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf %TEMP%\LLVM-7.0.0-win64.exe https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/LLVM-7.0.0-win64.exe"
     set CLANG_DIR=%CD%\citools\clang-rust
     %TEMP%\LLVM-7.0.0-win64.exe /S /NCRC /D=%CLANG_DIR%
     set RUST_CONFIGURE_ARGS=%RUST_CONFIGURE_ARGS% --set llvm.clang-cl=%CLANG_DIR%\bin\clang-cl.exe

.azure-pipelines/steps/install-sccache.yml

@@ -9,7 +9,7 @@ steps:
 
 - script: |
     md sccache
-    powershell -Command "iwr -outf sccache\sccache.exe https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/2018-04-26-sccache-x86_64-pc-windows-msvc"
+    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf sccache\sccache.exe https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/2018-04-26-sccache-x86_64-pc-windows-msvc"
     echo ##vso[task.prependpath]%CD%\sccache
   displayName: Install sccache (Windows)
   condition: and(succeeded(), eq(variables['Agent.OS'], 'Windows_NT'))

.azure-pipelines/steps/install-windows-build-deps.yml

@@ -54,9 +54,9 @@ steps:
 # Note that we don't literally overwrite the gdb.exe binary because it appears
 # to just use gdborig.exe, so that's the binary we deal with instead.
 - script: |
-    powershell -Command "iwr -outf %MINGW_ARCHIVE% %MINGW_URL%/%MINGW_ARCHIVE%"
+    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf %MINGW_ARCHIVE% %MINGW_URL%/%MINGW_ARCHIVE%"
     7z x -y %MINGW_ARCHIVE% > nul
-    powershell -Command "iwr -outf 2017-04-20-%MSYS_BITS%bit-gdborig.exe %MINGW_URL%/2017-04-20-%MSYS_BITS%bit-gdborig.exe"
+    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf 2017-04-20-%MSYS_BITS%bit-gdborig.exe %MINGW_URL%/2017-04-20-%MSYS_BITS%bit-gdborig.exe"
     mv 2017-04-20-%MSYS_BITS%bit-gdborig.exe %MINGW_DIR%\bin\gdborig.exe
     echo ##vso[task.prependpath]%CD%\%MINGW_DIR%\bin
   condition: and(succeeded(), eq(variables['Agent.OS'], 'Windows_NT'), ne(variables['MINGW_URL'],''))
@@ -81,7 +81,7 @@ steps:
 # Note that this is originally from the github releases patch of Ninja
 - script: |
     md ninja
-    powershell -Command "iwr -outf 2017-03-15-ninja-win.zip https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/2017-03-15-ninja-win.zip"
+    powershell -Command "$ProgressPreference = 'SilentlyContinue'; iwr -outf 2017-03-15-ninja-win.zip https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror/2017-03-15-ninja-win.zip"
     7z x -oninja 2017-03-15-ninja-win.zip
     del 2017-03-15-ninja-win.zip
     set RUST_CONFIGURE_ARGS=%RUST_CONFIGURE_ARGS% --enable-ninja