Set version to 20.1.0-rc1 (#125367)

Author: tstellar

cmake/Modules/LLVMVersion.cmake

@@ -10,6 +10,6 @@ if(NOT DEFINED LLVM_VERSION_PATCH)
   set(LLVM_VERSION_PATCH 0)
 endif()
 if(NOT DEFINED LLVM_VERSION_SUFFIX)
-  set(LLVM_VERSION_SUFFIX git)
+  set(LLVM_VERSION_SUFFIX -rc1)
 endif()