[clang] Fix typos in the default logic for CLANG_DEFAULT_UNWINDLIB

CLANG_DEFAULT_RTLIB had a typo, and libunwind isn't a valid
option for it.

This keeps the actual behaviour from before, defaulting to none if
using compiler-rt as rtlib.

Differential Revision: https://reviews.llvm.org/D98022

clang/CMakeLists.txt

@@ -262,7 +262,7 @@ set(CLANG_DEFAULT_UNWINDLIB "" CACHE STRING
 if (CLANG_DEFAULT_UNWINDLIB STREQUAL "")
   if (CLANG_DEFAULT_RTLIB STREQUAL "libgcc")
     set (CLANG_DEFAULT_UNWINDLIB "libgcc" CACHE STRING "" FORCE)
-  elseif (CLANG_DEFAULT_RTLIBS STREQUAL "libunwind")
+  elseif (CLANG_DEFAULT_RTLIB STREQUAL "compiler-rt")
     set (CLANG_DEFAULT_UNWINDLIB "none" CACHE STRING "" FORCE)
   endif()
 endif()