Revert "[clang][AST] Support AST files larger than 512M"

Bitcode file alignment is only 32-bit so 64-bit offsets need
special handling.
/b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:6327:28: runtime error: load of misaligned address 0x7fca2bcfe54c for type 'const uint64_t' (aka 'const unsigned long'), which requires 8 byte alignment
0x7fca2bcfe54c: note: pointer points here
  00 00 00 00 5a a6 01 00  00 00 00 00 19 a7 01 00  00 00 00 00 48 a7 01 00  00 00 00 00 7d a7 01 00
              ^
    #0 0x3be2fe4 in clang::ASTReader::TypeCursorForIndex(unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:6327:28
    #1 0x3be30a0 in clang::ASTReader::readTypeRecord(unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:6348:24
    #2 0x3bd3d4a in clang::ASTReader::GetType(unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:6985:26
    #3 0x3c5d9ae in clang::ASTDeclReader::Visit(clang::Decl*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:533:31
    #4 0x3c91cac in clang::ASTReader::ReadDeclRecord(unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4045:10
    #5 0x3bd4fb1 in clang::ASTReader::GetDecl(unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:7352:5
    #6 0x3bce2f9 in clang::ASTReader::ReadASTBlock(clang::serialization::ModuleFile&, unsigned int) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:3625:22
    #7 0x3bd6d75 in clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, llvm::SmallVectorImpl<clang::ASTReader::ImportedSubmodule>*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Serialization/ASTReader.cpp:4230:32
    #8 0x3a6b415 in clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, llvm::StringRef, bool, bool, clang::Preprocessor&, clang::InMemoryModuleCache&, clang::ASTContext&, clang::PCHContainerReader const&, llvm::ArrayRef<std::shared_ptr<clang::ModuleFileExtension> >, llvm::ArrayRef<std::shared_ptr<clang::DependencyCollector> >, void*, bool, bool, bool) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:539:19
    #9 0x3a6b00e in clang::CompilerInstance::createPCHExternalASTSource(llvm::StringRef, bool, bool, void*, bool) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:501:18
    #10 0x3abac80 in clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Frontend/FrontendAction.cpp:865:12
    #11 0x3a6e61c in clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:972:13
    #12 0x3ba74bf in clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:282:25
    #13 0xa3f753 in cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/tools/driver/cc1_main.cpp:240:15
    #14 0xa3a68a in ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/tools/driver/driver.cpp:330:12
    #15 0xa37f31 in main /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/tools/driver/driver.cpp:407:12
    #16 0x7fca2a7032e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #17 0xa21029 in _start (/b/sanitizer-x86_64-linux-fast/build/llvm_build_ubsan/bin/clang-11+0xa21029)

This reverts commit 30d5946d.