Bug 1512822 - Use clang-cl rather than MSVC for aarch64 builds r=chmanchester

Differential Revision: https://phabricator.services.mozilla.com/D17021

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -843,11 +843,7 @@ def default_c_compilers(host_or_target, other_c_compiler=None):
     def default_c_compilers(host_or_target, target, toolchain_prefix,
                             android_clang_compiler, *other_c_compiler):
         if host_or_target.kernel == 'WINNT':
-            # Prefer MSVC for aarch64 for now.
-            if host_or_target.cpu == 'aarch64':
-                supported = types = ('msvc', 'clang-cl', 'gcc', 'clang')
-            else:
-                supported = types = ('clang-cl', 'msvc', 'gcc', 'clang')
+            supported = types = ('clang-cl', 'msvc', 'gcc', 'clang')
         elif host_or_target.kernel == 'Darwin':
             types = ('clang',)
             supported = ('clang', 'gcc')

build/win64-aarch64/mozconfig.vs-latest

@@ -1 +1,3 @@
 . $topsrcdir/build/win64-aarch64/mozconfig.vs2017
+. "$topsrcdir/build/mozconfig.clang-cl"
+. "$topsrcdir/build/mozconfig.lld-link"

build/win64-aarch64/mozconfig.vs2017

@@ -20,15 +20,7 @@ if [ -d "${VSPATH}" ]; then
 
     # We need to declare host and target tools separately.
     arm_bin="${VSPATH}/VC/bin/Hostx64/arm64"
-    export CC="${arm_bin}/cl.exe"
-    export CXX="${arm_bin}/cl.exe"
     export AS="${arm_bin}/armasm64.exe"
-    export LINKER="${arm_bin}/link.exe"
-
-    x64_bin="${VSPATH}/VC/bin/Hostx64/x64"
-    export HOST_CC="${x64_bin}/cl.exe"
-    export HOST_CXX="${x64_bin}/cl.exe"
-    export HOST_LINKER="${x64_bin}/link.exe"
 
     # We provided LIB, above, but we also need to provide HOST_LDFLAGS so host
     # links are not completely confused.  LIBPATH wants its argument with