Bug 1748056 - Stop forcing c++14 mode when building clang on Windows. r=firefox-build-system-reviewers,andi

Back when this was added, clang's default mode might have been c++11,
but it's not c++17, and we're effectively downgrading it (which
incidentally causes problems to build libc++).

Differential Revision: https://phabricator.services.mozilla.com/D137255

build/build-clang/build-clang.py

@@ -775,8 +775,6 @@ if __name__ == "__main__":
         extra_cflags2 = []
         extra_cxxflags2 = [
             "-fms-compatibility-version=19.15.26726",
-            "-Xclang",
-            "-std=c++14",
         ]
         extra_asmflags = []
         extra_ldflags = []