gn build: use better triple on windows

The CMake build uses "x86_64-pc-windows-msvc". The "-msvc" suffix is
important because e.g. clang/test/lit.cfg.py matches against the
suffix "windows-msvc" to compute the presence of the "ms-sdk" and
the absence of the "LP64" feature.

Differential Revision: https://reviews.llvm.org/D68572

llvm-svn: 373899
diff --git a/llvm/utils/gn/secondary/llvm/triples.gni b/llvm/utils/gn/secondary/llvm/triples.gni
index efe8be2..5587312 100644
--- a/llvm/utils/gn/secondary/llvm/triples.gni
+++ b/llvm/utils/gn/secondary/llvm/triples.gni
@@ -10,7 +10,7 @@
   } else if (current_os == "mac") {
     llvm_current_triple = "x86_64-apple-darwin"
   } else if (current_os == "win") {
-    llvm_current_triple = "x86_64-pc-windows"
+    llvm_current_triple = "x86_64-pc-windows-msvc"
   }
 } else if (current_cpu == "arm64") {
   if (current_os == "android") {