Bug 1651305 - Simplify how USE_INTEGRATED_CLANGCL_AS is handled. r=froydnj

The current setup sets /some/ flags via CLANGCL_ASFLAGS (handling of x86
is notably missing, for instance), and uses "clang-cl" as the assembler,
assuming it's in $PATH.

This can be simplified by just using `CC`, which will contain the full
path to "clang-cl" and the right flags for the targets, which makes
CLANGCL_ASFLAGS unnecessary.

Differential Revision: https://phabricator.services.mozilla.com/D82660

build/moz.configure/toolchain.configure

@@ -1784,19 +1784,6 @@ set_config('HAVE_YASM', have_yasm)
 add_old_configure_assignment('YASM', have_yasm)
 
 
-# clang-cl integrated assembler support
-# ==============================================================
-@depends(target)
-def clangcl_asflags(target):
-    asflags = None
-    if target.os == 'WINNT' and target.cpu == 'aarch64':
-        asflags = ['--target=aarch64-windows-msvc']
-    return asflags
-
-
-set_config('CLANGCL_ASFLAGS', clangcl_asflags)
-
-
 # Code Coverage
 # ==============================================================
 

python/mozbuild/mozbuild/frontend/emitter.py

@@ -1411,14 +1411,11 @@ class TreeMetadataEmitter(LoggingMixin):
                                             context.config.substs.get('NASM_ASFLAGS', []))
 
         if context.get('USE_INTEGRATED_CLANGCL_AS') is True:
-            clangcl = context.config.substs.get('CLANG_CL')
-            if not clangcl:
+            if context.config.substs.get('CC_TYPE') != 'clang-cl':
                 raise SandboxValidationError('clang-cl is not available', context)
-            passthru.variables['AS'] = 'clang-cl'
+            passthru.variables['AS'] = context.config.substs.get('CC')
             passthru.variables['AS_DASH_C_FLAG'] = '-c'
             passthru.variables['ASOUTOPTION'] = '-o '
-            computed_as_flags.resolve_flags('OS',
-                                            context.config.substs.get('CLANGCL_ASFLAGS', []))
 
         if passthru.variables:
             yield passthru