[clang] Don't set CLANG_DEFAULT_UNWINDLIB to none if rtlib is set to compiler-rt

[mstorsjo]

Summary

002dd47bdd674fad8186650f07458b1e062545df was meant to not be any
functional change, but it turned out it was.

With CLANG_DEFAULT_RTLIB set to compiler-rt, CLANG_DEFAULT_UNWINDLIB used
to bet set to an empty string, but now was set to "none".

If one only overrode rtlib to libgcc, one previously would get libgcc
as unwind lib, but now didn't. This caused test failures, fixed in
41476d89b82647c1ff690fdc805c859262d571e5.

Secondly, for the android target, the previous default was to link
libunwind, which this now changed.

Reinstate the exact same behaviour as before (removing the previously
typoed cmake check) and fix the option comment in one place to match
the other one above.

Or shoulf we just remove the default-inferring logic (currently only
setting unwindlib=libgcc) altogether, as the simpler default (empty
string, equal to -unwindlib=platform) does the same anyway?

[MaskRay]

The merit is less CMake magic.

[phosek]

LGTM

clang/CMakeLists.txt

set(CLANG_DEFAULT_RTLIB "" CACHE STRING
     "Default runtime library to use (\"libgcc\" or \"compiler-rt\", empty for platform default)" FORCE)
 endif()
 
 set(CLANG_DEFAULT_UNWINDLIB "" CACHE STRING
   "Default unwind library to use (\"none\" \"libgcc\" or \"libunwind\", empty to match runtime library.)")
 if (CLANG_DEFAULT_UNWINDLIB STREQUAL "")
   if (CLANG_DEFAULT_RTLIB STREQUAL "libgcc")
     set (CLANG_DEFAULT_UNWINDLIB "libgcc" CACHE STRING "" FORCE)
-  elseif (CLANG_DEFAULT_RTLIB STREQUAL "compiler-rt")
-    set (CLANG_DEFAULT_UNWINDLIB "none" CACHE STRING "" FORCE)
   endif()
 endif()
 
 if (NOT(CLANG_DEFAULT_UNWINDLIB STREQUAL "" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "none" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "libgcc" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "libunwind"))
   message(WARNING "Resetting default unwindlib to use platform default")
   set(CLANG_DEFAULT_UNWINDLIB "" CACHE STRING
-    "Default unwind library to use (\"none\" \"libgcc\" or \"libunwind\", empty for none)" FORCE)
+    "Default unwind library to use (\"none\" \"libgcc\" or \"libunwind\", empty to match runtime library.)" FORCE)
 endif()
 
 set(CLANG_DEFAULT_OBJCOPY "objcopy" CACHE STRING
   "Default objcopy executable to use.")
 
 set(CLANG_DEFAULT_OPENMP_RUNTIME "libomp" CACHE STRING
   "Default OpenMP runtime used by -fopenmp.")