Bug 1680826 - Update MSVC compat version in build-clang.py r=firefox-build-system-reviewers,mhentges

In https://reviews.llvm.org/D92515, clang-12 bumps the build requirement to MSVC version 19.14.

We've already updated well past that (most recently bug 1485224) but build-clang.py never got the memo.

Differential Revision: https://phabricator.services.mozilla.com/D98806

build/build-clang/build-clang.py

@@ -862,7 +862,7 @@ if __name__ == "__main__":
         # Force things on.
         extra_cflags2 = []
         extra_cxxflags2 = [
-            "-fms-compatibility-version=19.13.26128",
+            "-fms-compatibility-version=19.15.26726",
             "-Xclang",
             "-std=c++14",
         ]