Bug 1569728 - build clang toolchains with GCC 7; r=mshal

We need this change so that the newly-built clang will have
C++17-compatible libstdc++ headers installed. I believe this change also
means that the newly-built clang (and associated tools) links against
GCC 7's libstdc++, but we set RPATH or similar appropriately, so there
shouldn't be issues stemming from that.

MANUAL PUSH: toolchain patches are requested to land on inbound

Differential Revision: https://phabricator.services.mozilla.com/D41251

taskcluster/ci/toolchain/clang.yml

@@ -26,7 +26,7 @@ linux64-clang-4.0:
         fetch:
             - clang-4.0
         toolchain:
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-7:
     description: "Clang 7 toolchain build"
@@ -44,7 +44,7 @@ linux64-clang-7:
         fetch:
             - clang-7
         toolchain:
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-9:
     description: "Clang 9 toolchain build"
@@ -64,7 +64,7 @@ linux64-clang-9:
             - clang-9
         toolchain:
             - linux64-binutils
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-9-cross:
     description: "Clang 9 toolchain with patches to cross-compile to windows"
@@ -84,7 +84,7 @@ linux64-clang-9-cross:
             - clang-9
         toolchain:
             - linux64-binutils
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-8-mingw-x86:
     description: "MinGW-Clang Trunk x86 toolchain build"
@@ -106,7 +106,7 @@ linux64-clang-8-mingw-x86:
             - libunwind
             - llvm-mingw
         toolchain:
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-8-mingw-x64:
     description: "MinGW-Clang Trunk x64 toolchain build"
@@ -129,7 +129,7 @@ linux64-clang-8-mingw-x64:
             - libunwind
             - llvm-mingw
         toolchain:
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-9-android-cross:
     description: "Clang 9 toolchain build"
@@ -149,7 +149,7 @@ linux64-clang-9-android-cross:
             - clang-9
         toolchain:
             - linux64-binutils
-            - linux64-gcc-6
+            - linux64-gcc-7
             - linux64-android-ndk-linux-repack
 
 linux64-clang-9-aarch64-cross:
@@ -174,7 +174,7 @@ linux64-clang-9-aarch64-cross:
             - clang-9
         toolchain:
             - linux64-binutils
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 linux64-clang-9-macosx-cross:
     description: "Clang 9 toolchain build with MacOS Compiler RT libs"
@@ -202,7 +202,7 @@ linux64-clang-9-macosx-cross:
             - linux64-binutils
             - linux64-cctools-port
             - linux64-clang-9
-            - linux64-gcc-6
+            - linux64-gcc-7
 
 macosx64-clang:
     description: "Clang toolchain build"
@@ -232,7 +232,7 @@ macosx64-clang:
             - linux64-binutils
             - linux64-cctools-port
             - linux64-clang-9
-            - linux64-gcc-6
+            - linux64-gcc-7
             - linux64-node
 
 win64-clang-cl: