Add support for Microsoft __if_exists and __if_not_exists construct inside function definition.

Allow to include or exclude code depending on if a symbol exists or not. Just like a #ifdef but for C/C++ symbols.

More doc: http://msdn.microsoft.com/en-us/library/x7wy9xh3(v=VS.100).aspx

Support at class and namespace scopes will be added later.

llvm-svn: 131014