Correct PrivateLinkage for COFF

- Use storage class C_STAT for 'PrivateLinkage' The storage class for
  PrivateLinkage should equal to the Internal Linkage.

- Set 'PrivateGlobalPrefix' from "L" to ".L" for MM_WinCOFF (includes
  x86_64) MM_WinCOFF has empty GlobalPrefix '\0' so PrivateGlobalPrefix
  "L" may conflict to the normal symbol name starting with 'L'.

Based on a patch by Han Sangjin! Manually updated test cases.

llvm-svn: 284096