Windows: Use GetModuleHandleEx instead of LoadLibrary

We were using an anti-pattern of:
 - LoadLibrary
 - GetProcAddress
 - FreeLibrary

This is problematic because of several reasons:
 - We are holding on to pointers into a library we just unloaded.
 - Calling LoadLibrary results in an increase in the reference count of
   the library in question and any libraries that it depends on and
   so-on and so-forth.  This is none too quick.

Instead, use GetModuleHandleEx with GET_MODULE_HANDLE_EX_FLAG_PIN.  This
is done because because we didn't bring the reference for the library
into existence and therefor shouldn't count on it being around later.

llvm-svn: 192550