Fix check-llvm on kernel 4.9+ with asan or msan

[vitalybuka]

Summary

Before https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?h=v4.9.46&id=84638335900f1995495838fe1bd4870c43ec1f67
test worked because memory allocated with mmap was not counted against RLIMIT_DATA.

[vitalybuka]

Update doc string

[eugenis]

LGTM with TSan

tools/bugpoint/bugpoint.cpp

172

what about TSan?

[vitalybuka]

Added tsan

tools/bugpoint/bugpoint.cpp

#endif
 
   if (MemoryLimit < 0) {
     // Set the default MemoryLimit.  Be sure to update the flag's description if
     // you change this.
     if (sys::RunningOnValgrind() || UseValgrind)
       MemoryLimit = 800;
     else
       MemoryLimit = 400;
+#if LLVM_ADDRESS_SANITIZER_BUILD || LLVM_MEMORY_SANITIZER_BUILD
+    // Starting from kernel 4.9 memory allocated with mmap is counted against
+    // RLIMIT_DATA. Sanitizers need to allocate tens of terabytes for shadow.
+    MemoryLimit = 0;
+#endif
   }
 
   BugDriver D(argv[0], FindBugs, TimeoutValue, MemoryLimit, UseValgrind,
               Context);
   if (D.addSources(InputFilenames))
     return 1;
 
   AddToDriver PM(D);