Fix strlen() of unbound array undefined behavior

LLDB testsuite fails when built by GCC8 on:
  LLDB :: SymbolFile/DWARF/find-basic-namespace.cpp

This is because this code in LLDB codebase has undefined behavior:

  #include <algorithm>
  #include <string.h>
  // lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp:1731
  static struct section_64 {
    char sectname[16];
    char segname[16];
  } sect64 = { {'_','_','a','p','p','l','e','_','n','a','m','e','s','p','a','c'}, "__DWARF" };
  int main() {
    return std::min<size_t>(strlen(sect64.sectname), sizeof(sect64.sectname));
  }

It has been discussed as a (false) bugreport to GCC:
  wrong-code: LLDB testcase fails: SymbolFile/DWARF/find-basic-namespace.cpp
  https://bugzilla.redhat.com/show_bug.cgi?id=1672436

Differential Revision: https://reviews.llvm.org/D57781

llvm-svn: 353280