gitlab-ci: Test against LLVM / clang 9 on x86

They're not available for Debian buster yet, so we have to use upstream
snapshot packages again.

In contrast to earlier, we now store the LLVM APT repository key in Git
instead of re-downloading it every time.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index b7298bc..b358998 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -96,7 +96,7 @@
     - .debian@container-ifnot-exists
     - .container
   variables:
-    DEBIAN_TAG: &x86_build "2019-11-13"
+    DEBIAN_TAG: &x86_build "2020-01-07"
 
 .use-x86_build:
   variables:
@@ -109,13 +109,13 @@
 x86_test-gl:
   extends: x86_build
   variables:
-    DEBIAN_TAG: &x86_test-gl "2019-12-18"
+    DEBIAN_TAG: &x86_test-gl "2020-01-07"
 
 # Debian 10 based x86 test image for VK
 x86_test-vk:
   extends: x86_build
   variables:
-    DEBIAN_TAG: &x86_test-vk "2019-12-18"
+    DEBIAN_TAG: &x86_test-vk "2020-01-07"
   # Can only be triggered manually on personal branches because RADV is the only
   # driver that does Vulkan testing at the moment.
   rules:
@@ -215,6 +215,8 @@
   extends:
     - .build-linux
     - .use-x86_build
+  variables:
+    LLVM_VERSION: 9
   script:
     - .gitlab-ci/meson-build.sh
 
@@ -268,7 +270,6 @@
       -D gallium-nine=true
       -D gallium-opencl=disabled
     GALLIUM_DRIVERS: "iris,nouveau,kmsro,r300,r600,freedreno,swr,swrast,svga,v3d,vc4,virgl,etnaviv,panfrost,lima,zink"
-    LLVM_VERSION: "7"
     EXTRA_OPTION: >
       -D osmesa=gallium
       -D tools=all
@@ -320,6 +321,7 @@
     - .ci-deqp-artifacts
   variables:
     BUILDTYPE: "debugoptimized"
+    LLVM_VERSION: "8"
     VULKAN_DRIVERS: "freedreno,amd"
   script:
     - .gitlab-ci/meson-build.sh
@@ -334,8 +336,8 @@
     DRI_DRIVERS: "auto"
     GALLIUM_DRIVERS: "auto"
     VULKAN_DRIVERS: intel,amd,freedreno
-    CC: "ccache clang-8"
-    CXX: "ccache clang++-8"
+    CC: "ccache clang-9"
+    CXX: "ccache clang++-9"
 
 .meson-windows:
   extends:
@@ -425,7 +427,6 @@
       -D cpp_args=-fno-sanitize-recover=all
     UBSAN_OPTIONS: "print_stacktrace=1"
     VULKAN_DRIVERS: intel,amd,freedreno
-    LLVM_VERSION: "8"
     EXTRA_OPTION: >
       -D vulkan-overlay-layer=true
 
@@ -462,7 +463,7 @@
     SCONS_CHECK_COMMAND: "scons llvm=1 force_scons=1 check"
   script:
     - SCONS_TARGET="" SCONS_CHECK_COMMAND="scons check force_scons=1" .gitlab-ci/scons-build.sh
-    - LLVM_VERSION=8 .gitlab-ci/scons-build.sh
+    - LLVM_VERSION=9 .gitlab-ci/scons-build.sh
 
 scons-old-llvm:
   extends: