Define __STDC__ in -fms-extensions mode; defining it is consistent with gcc,...

Define __STDC__ in -fms-extensions mode; defining it is consistent with gcc, and not defining it causes the system headers on multiple platforms to break with -fms-extensions because they assume the lack of a definition of __STDC__ implies a traditional (pre-ANSI) preprocessor. PR10324 and <rdar://problem/10313809> .

llvm-svn: 142554