ADT: Remove AlignedCharArrayUnion, NFC

Prep commit already migrated users over to std::aligned_union_t; this
just deletes the type / header / test.

Differential Revision: https://reviews.llvm.org/D92517

Author: dexonsmith

llvm/include/llvm/Support/AlignOf.h

@@ -4,7 +4,6 @@ set(LLVM_LINK_COMPONENTS
 
 add_llvm_unittest(SupportTests
   AlignmentTest.cpp
-  AlignOfTest.cpp
   AllocatorTest.cpp
   AnnotationsTest.cpp
   ARMAttributeParser.cpp