[Sanitizers] Provide __internal_dup on Solaris

[Sanitizers] Provide __internal_dup on Solaris

Both LLVM 8.0.0 and current trunk fail to link libclang_rt.ubsan.standalone-*.so
on Solaris 11/x86 with GCC 8.1.0:

  Undefined			first referenced
   symbol  			    in file
  Scanning dependencies of target clang_rt.asan_cxx-x86_64
  __sanitizer::internal_dup(int)      ../sanitizer_common/CMakeFiles/RTSanitizerCommon.i386.dir/sanitizer_posix.cc.o
  ld: fatal: symbol referencing errors
  collect2: error: ld returned 1 exit status
  make[2]: *** [projects/compiler-rt/lib/ubsan/CMakeFiles/clang_rt.ubsan_standalone-dynamic-i386.dir/build.make:233: lib/clang/9.0.0/lib/sunos/libclang_rt.ubsan_standalone-i386.so] Error 1

Fixed by providing an implementation, which allowed the i386-pc-solaris2.11 build
to finish.

Differential Revision: https://reviews.llvm.org/D60045

llvm-svn: 357753