Bump version to 21.1.0-rc1

cmake/Modules/LLVMVersion.cmake

diff --git a/cmake/Modules/LLVMVersion.cmake b/cmake/Modules/LLVMVersion.cmake
index c12240f9..ef12515 100644
--- a/cmake/Modules/LLVMVersion.cmake
+++ b/cmake/Modules/LLVMVersion.cmake
@@ -10,6 +10,6 @@
   set(LLVM_VERSION_PATCH 0)
 endif()
 if(NOT DEFINED LLVM_VERSION_SUFFIX)
-  set(LLVM_VERSION_SUFFIX git)
+  set(LLVM_VERSION_SUFFIX -rc1)
 endif()