Remove const qualifier from Modules returned by ExternalASTSource. (NFC)

This API is used by LLDB to attach owning module information to
Declarations deserialized from DWARF.

Differential Revision: https://reviews.llvm.org/D75561
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 7437f64..bea9bdd 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -8514,7 +8514,7 @@
 
 llvm::Optional<ASTSourceDescriptor>
 ASTReader::getSourceDescriptor(unsigned ID) {
-  if (const Module *M = getSubmodule(ID))
+  if (Module *M = getSubmodule(ID))
     return ASTSourceDescriptor(*M);
 
   // If there is only a single PCH, return it instead.