gn build: use better triple on windows

[thakis]

Summary

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.

llvm/trunk/utils/gn/secondary/llvm/triples.gni

 if (current_cpu == "x86") {
   if (current_os == "linux") {
     llvm_current_triple = "i386-unknown-linux-gnu"
   }
 } else if (current_cpu == "x64") {
   if (current_os == "freebsd") {
     llvm_current_triple = "x86_64-unknown-freebsd"
   } else if (current_os == "linux") {
     llvm_current_triple = "x86_64-unknown-linux-gnu"
   } 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") {
     llvm_current_triple = "aarch64-linux-android21"
   }
 } else if (current_cpu == "ppc64") {
   if (current_os == "linux") {
     llvm_current_triple = "powerpc64le-unknown-linux-gnu"