Bug 1480631 - Switch Linux builds to clang. r=froydnj

This change switches most CI builds to clang, with a few exceptions:
- valgrind builds, until bug 1481670 is figured out.
- PGO and nightly builds, until that's fully tested.
- coverage builds, per bug 1471339 comment 17.
- base toolchain builds, to keep some builds on GCC even when we're
  fully switched to clang.
- any build that doesn't use build/unix/mozconfig.linux (e.g. probably
  all those driven by autospider.py, maybe others).

build/unix/mozconfig.linux

@@ -7,9 +7,13 @@ fi
 
 TOOLTOOL_DIR=${TOOLTOOL_DIR:-$topsrcdir}
 
-# We deal with valgrind builds here
-CC="$TOOLTOOL_DIR/gcc/bin/gcc"
-CXX="$TOOLTOOL_DIR/gcc/bin/g++"
+if [ -n "$FORCE_GCC" ]; then
+    CC="$TOOLTOOL_DIR/gcc/bin/gcc"
+    CXX="$TOOLTOOL_DIR/gcc/bin/g++"
+else
+    CC="$TOOLTOOL_DIR/clang/bin/clang"
+    CXX="$TOOLTOOL_DIR/clang/bin/clang++"
+fi
 
 # We want to make sure we use binutils and other binaries in the tooltool
 # package.

taskcluster/ci/build/linux.yml

@@ -107,6 +107,8 @@ linux64/pgo:
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -140,6 +142,7 @@ linux64-fuzzing/debug:
         max-run-time: 3600
         env:
             PERFHERDER_EXTRA_OPTIONS: fuzzing
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -240,6 +243,8 @@ linux64-devedition-nightly/opt:
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -275,6 +280,7 @@ linux64-base-toolchains/opt:
         max-run-time: 7200
         env:
             PERFHERDER_EXTRA_OPTIONS: base-toolchains
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -306,6 +312,7 @@ linux64-base-toolchains/debug:
         max-run-time: 3600
         env:
             PERFHERDER_EXTRA_OPTIONS: base-toolchains
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -406,6 +413,8 @@ linux/pgo:
     worker:
         docker-image: {in-tree: debian7-i386-build}
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -516,6 +525,8 @@ linux-devedition-nightly/opt:
     worker:
         docker-image: {in-tree: debian7-i386-build}
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -556,6 +567,8 @@ linux-nightly/opt:
     worker:
         docker-image: {in-tree: debian7-i386-build}
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -831,6 +844,8 @@ linux64-nightly/opt:
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -1033,6 +1048,8 @@ linux64-ccov/debug:
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]
@@ -1065,6 +1082,8 @@ linux64-ccov/opt:
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         max-run-time: 7200
+        env:
+            FORCE_GCC: '1'
     run:
         using: mozharness
         actions: [get-secrets build check-test update]

taskcluster/ci/valgrind/kind.yml

@@ -31,6 +31,7 @@ jobs:
             max-run-time: 72000
             env:
                 PERFHERDER_EXTRA_OPTIONS: valgrind
+                FORCE_GCC: '1'
         run:
             using: mozharness
             actions: [get-secrets build valgrind-test]