[Glibc] Add missing C stdlib header

`assert.h` is missing from the modulemap, which causes clang to consider `assert.h` to be a part of the first module to include it. This causes issues in SwiftCompilerSources when we try to use classes coming from LLVM headers on Linux.

Author: egorzhdan

stdlib/public/Platform/glibc.modulemap.gyb

@@ -42,6 +42,10 @@ module SwiftGlibc [system] {
 % end
 
   header "SwiftGlibc.h"
+
+  // <assert.h>'s use of NDEBUG requires textual inclusion. 
+  textual header "assert.h"
+
   export *
 }