NFC: Fix some darwin linker warnings introduced in r338385

The darwin linker was complaining about Toolchains/RISCV.cpp and
Toolchains/Arch/RISCV.cpp had the same name. Fix is to just rename
Toolchains/RISCV.cpp to Toolchains/RISCVToolchain.cpp.

Differential revision: https://reviews.llvm.org/D52574

llvm-svn: 343263
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 8266f92..d356983 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -37,7 +37,7 @@
 #include "ToolChains/NetBSD.h"
 #include "ToolChains/OpenBSD.h"
 #include "ToolChains/PS4CPU.h"
-#include "ToolChains/RISCV.h"
+#include "ToolChains/RISCVToolchain.h"
 #include "ToolChains/Solaris.h"
 #include "ToolChains/TCE.h"
 #include "ToolChains/WebAssembly.h"