Handle function name conflicts in _LIBCPP_MSVCRT mode

Visual Studio's SAL extension uses a macro named __deallocate. This macro is
used pervasively, and gets included through various different ways. This
conflicts with the similarly named interfaces in libc++. Introduce a undef
header similar to __undef_min_max to handle this. This fixes a number of errors
due to the macro replacing the function name.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@229162 91177308-0d34-0410-b5e6-96231b3b80d8

include/module.modulemap

@@ -469,4 +469,5 @@ module std [system] {
   module __tree { header "__tree" export * }
   module __tuple { header "__tuple" export * }
   module __undef_min_max { header "__undef_min_max" export * }
+  module __undef___deallocate { header "__undef___deallocate" export * }
 }