gitlab-ci: Build clover against all supported versions of LLVM

And consolidate it all into a single job.

It doesn't take much longer than a single version, thanks to ccache.
Overall, this single job might be faster or at least use fewer CPU
cycles than the two jobs before, while covering thrice as many versions
of LLVM.

v2:
* Move "rm -rf _build" to meson-build.sh.
* Set GALLIUM_DRIVERS the same way both times in the meson-clover job,
  for symmetry.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> # v1
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 1b448b3..de6e7d1 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -14,7 +14,7 @@
 # repository's registry will be used there as well.
 variables:
   UPSTREAM_REPO: mesa/mesa
-  DEBIAN_TAG: "2019-05-01"
+  DEBIAN_TAG: "2019-05-08"
   DEBIAN_VERSION: stretch-slim
   DEBIAN_IMAGE: "$CI_REGISTRY_IMAGE/debian/$DEBIAN_VERSION:$DEBIAN_TAG"
 
@@ -183,7 +183,7 @@
     GALLIUM_DRIVERS: "iris,nouveau,kmsro,r300,r600,freedreno,svga,v3d,vc4,virgl,etnaviv,panfrost,lima"
     LLVM_VERSION: "7"
 
-meson-clover-llvm:
+meson-clover:
   extends: .meson-build
   variables:
     UNWIND: "true"
@@ -200,13 +200,15 @@
       -D gallium-xa=false
       -D gallium-nine=false
       -D gallium-opencl=icd
-    GALLIUM_DRIVERS: "r600,radeonsi"
-
-meson-clover-llvm39:
-  extends: meson-clover-llvm
-  variables:
-    GALLIUM_DRIVERS: "i915,r600"
-    LLVM_VERSION: "3.9"
+  script:
+    - export GALLIUM_DRIVERS="r600,radeonsi"
+    - .gitlab-ci/meson-build.sh
+    - LLVM_VERSION=7 .gitlab-ci/meson-build.sh
+    - export GALLIUM_DRIVERS="i915,r600"
+    - LLVM_VERSION=3.9 .gitlab-ci/meson-build.sh
+    - LLVM_VERSION=4.0 .gitlab-ci/meson-build.sh
+    - LLVM_VERSION=5.0 .gitlab-ci/meson-build.sh
+    - LLVM_VERSION=6.0 .gitlab-ci/meson-build.sh
 
 scons-nollvm:
   extends: .scons-build