Mark memrchr as unavailable on Windows

[mati865]

Summary

Otherwise LLVM will optimise strrchr into memrchr on Windows resulting in linker error:

$ cat memrchr_test.c
#include <string.h>
int main(int argc, char **argv) {
    return (long)strrchr("KkMm", argv[argc-1][0]);
}

$ clang memrchr_test.c -O
memrchr_test.c:3:12: warning: cast to smaller integer type 'long' from 'char *' [-Wpointer-to-int-cast]
    return (long)strrchr("KkMm", argv[argc-1][0]);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
ld.lld: error: undefined symbol: memrchr
>>> referenced by D:/msys64/tmp/memrchr_test-e7aabd.o:(main)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Example taken from MSYS2 Discord and tested with windows-gnu target.

[aeubanks]

lg

I was going to ask for a test but it looks like we don't really test per-target available libcalls and that does seem tedious and probably not worth it

[mati865]

Thank you.

llvm/lib/Analysis/TargetLibraryInfo.cpp

if (T.isOSWindows() && !T.isWindowsCygwinEnvironment()) {
     TLI.setUnavailable(LibFunc_funlockfile);
     TLI.setUnavailable(LibFunc_getitimer);
     TLI.setUnavailable(LibFunc_getlogin_r);
     TLI.setUnavailable(LibFunc_getpwnam);
     TLI.setUnavailable(LibFunc_htonl);
     TLI.setUnavailable(LibFunc_htons);
     TLI.setUnavailable(LibFunc_lchown);
     TLI.setUnavailable(LibFunc_lstat);
+    TLI.setUnavailable(LibFunc_memrchr);
     TLI.setUnavailable(LibFunc_ntohl);
     TLI.setUnavailable(LibFunc_ntohs);
     TLI.setUnavailable(LibFunc_pread);
     TLI.setUnavailable(LibFunc_pwrite);
     TLI.setUnavailable(LibFunc_readlink);
     TLI.setUnavailable(LibFunc_realpath);
     TLI.setUnavailable(LibFunc_setitimer);
     TLI.setUnavailable(LibFunc_statvfs);