Bug 1693288 - Move linux64-ccov builds to clang 12 rc2 r=marco

Switching linux64-ccov builds to clang from gcc in bug 1499663 uncovered some performance problems that lead to test timeouts. These are addressed in clang 12. In order to ease the starring burden on sheriffs, let's give those builds a sneak preview of clang 12 even though general builds are not yet switching over.

Differential Revision: https://phabricator.services.mozilla.com/D107104

browser/config/mozconfigs/linux64/code-coverage-debug

@@ -2,3 +2,8 @@
 
 ac_add_options --enable-debug=-g1
 ac_add_options --enable-rust-tests
+
+# https://bugs.llvm.org/show_bug.cgi?id=49226
+# LLD 12.0.0 had a behavior change that breaks coverage builds.
+# Passing --no-fortran-common restores the old behavior.
+export LDFLAGS="$LDFLAGS -Wl,--no-fortran-common"

browser/config/mozconfigs/linux64/code-coverage-opt

@@ -2,3 +2,8 @@
 
 ac_add_options --enable-debug-symbols=-g1
 ac_add_options --enable-rust-tests
+
+# https://bugs.llvm.org/show_bug.cgi?id=49226
+# LLD 12.0.0 had a behavior change that breaks coverage builds.
+# Passing --no-fortran-common restores the old behavior.
+export LDFLAGS="$LDFLAGS -Wl,--no-fortran-common"

taskcluster/ci/build/linux.yml

@@ -1287,7 +1287,10 @@ linux64-ccov/opt:
     fetches:
         toolchain:
             - linux64-binutils
-            - linux64-clang
+            # For linux64-ccov only, start using the clang-12 toolchains ahead
+            # of general builds, due to bug 1692817. This can be reverted once
+            # clang-12 becomes the default (bug 1696917).
+            - linux64-clang-12
             - linux64-rust
             - linux64-cbindgen
             - linux64-dump-syms
@@ -1297,7 +1300,7 @@ linux64-ccov/opt:
             - linux64-grcov
             - linux64-lucetc
             - linux64-sysroot
-            - wasi-sysroot
+            - wasi-sysroot-12
 
 linux64-ccov/debug:
     description: "Linux64-CCov Debug"
@@ -1330,7 +1333,10 @@ linux64-ccov/debug:
     fetches:
         toolchain:
             - linux64-binutils
-            - linux64-clang
+            # For linux64-ccov only, start using the clang-12 toolchains ahead
+            # of general builds, due to bug 1692817. This can be reverted once
+            # clang-12 becomes the default (bug 1696917).
+            - linux64-clang-12
             - linux64-rust
             - linux64-cbindgen
             - linux64-dump-syms
@@ -1340,7 +1346,7 @@ linux64-ccov/debug:
             - linux64-grcov
             - linux64-lucetc
             - linux64-sysroot
-            - wasi-sysroot
+            - wasi-sysroot-12
 
 linux64-add-on-devel/opt:
     description: "Linux64 add-on-devel"