Avoid C++ comment in C sources

lib/Target/X86/Disassembler/X86DisassemblerDecoder.c:1361:7: error: C++ style comments are not allowed in ISO C90

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200153 91177308-0d34-0410-b5e6-96231b3b80d8