lib/Frontend/InitHeaderSearch.cpp: Fix compatibility to Cygming.

  - lib/gcc/include should not be included. Clang oughta have alternatives. (PR7956)
  - Cygwin: /usr/include/w32api should be included. gcc/cygwin does.
  - gcc/cygwin uses gcc paths as /usr/lib not /lib. They are same on Cygwin environment, though, We have to take what gcc/cygwin does.
  - Cygwin-1.7: Use 4.3.4.
  - Cygwin-1.5: Use 4.3.2 for gcc-4.

llvm-svn: 116182