[SystemZ] Support z15 processor name

The recently announced IBM z15 processor implements the architecture
already supported as "arch13" in LLVM.  This patch adds support for
"z15" as an alternate architecture name for arch13.

Corrsponding LLVM support was committed as rev. 372435.

llvm-svn: 372436
diff --git a/clang/lib/Basic/Targets/SystemZ.cpp b/clang/lib/Basic/Targets/SystemZ.cpp
index d86928a..ad3915e 100644
--- a/clang/lib/Basic/Targets/SystemZ.cpp
+++ b/clang/lib/Basic/Targets/SystemZ.cpp
@@ -92,7 +92,7 @@
   {{"arch10"}, 10}, {{"zEC12"}, 10},
   {{"arch11"}, 11}, {{"z13"}, 11},
   {{"arch12"}, 12}, {{"z14"}, 12},
-  {{"arch13"}, 13},
+  {{"arch13"}, 13}, {{"z15"}, 13}
 };
 
 int SystemZTargetInfo::getISARevision(StringRef Name) const {