Check for _strtoi64 in the cmake build if strtoll is missing

Previously this check was guarded by MSVC, which doesn't distinguish
between the compiler and the headers/library.  This enables clang to
compile more of LLVM on Windows with Microsoft headers.

Remove some unused macros while I'm here: error_t and LTDL stuff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187839 91177308-0d34-0410-b5e6-96231b3b80d8