Bug 1926651 - Point DXC builds to a newer revision.r=jimb,ErichDonGubler

Differential Revision: https://phabricator.services.mozilla.com/D250447

UltraBlame original commit: a207a86213e11644af224e6cb90de58524cc16ae

Author: marco-c

taskcluster/kinds/fetch/toolchains.yml

@@ -791,4 +791,4 @@ dxc-src:
     fetch:
         type: git
         repo: https://github.com/FirefoxGraphics/DirectXShaderCompiler
-        revision: c75bb05c0e040bc09ec012ccf9767b1ec622477f
+        revision: 81378be83a1690cac0e727941e1a963518d67c26