[ADT] PointerUnion::getAddrOfPtr1(): fix/silence -Wcast-qual warning.

Summary:
Previously, when D33102 landed, this broke -Werror buildbots.

http://lab.llvm.org:8011/builders/clang-with-lto-ubuntu/builds/3249
```
FAILED: /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/install/stage1/bin/clang++   -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_GLOBAL_ISEL -D_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Ilib/CodeGen/AsmPrinter -I/home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter -Iinclude -I/home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/include -fPIC -fvisibility-inlines-hidden -Werror -Werror=date-time -std=c++11 -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fcolor-diagnostics -ffunction-sections -fdata-sections -O3    -UNDEBUG  -fno-exceptions -fno-rtti -MMD -MT lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/CodeViewDebug.cpp.o -MF lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/CodeViewDebug.cpp.o.d -o lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/CodeViewDebug.cpp.o -c /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
In file included from /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp:14:
In file included from /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter/CodeViewDebug.h:17:
In file included from /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.h:15:
In file included from /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/include/llvm/IR/DebugInfoMetadata.h:26:
In file included from /home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/include/llvm/IR/Metadata.h:23:
/home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/include/llvm/ADT/PointerUnion.h:161:19: error: cast from 'void **' to 'const llvm::DISubprogram **' must have all intermediate pointers const qualified to be safe [-Werror,-Wcast-qual]
    return (PT1 *)Val.getAddrOfPointer();
                  ^
/home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/include/llvm/ADT/TinyPtrVector.h:177:18: note: in instantiation of member function 'llvm::PointerUnion<const llvm::DISubprogram *, llvm::SmallVector<const llvm::DISubprogram *, 4> *>::getAddrOfPtr1' requested here
      return Val.getAddrOfPtr1();
                 ^
/home/buildbot/Buildbot/Slave1a/clang-with-lto-ubuntu/llvm.src/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp:1885:33: note: in instantiation of member function 'llvm::TinyPtrVector<const llvm::DISubprogram *>::begin' requested here
    for (const DISubprogram *SP : MethodItr.second) {
                                ^
1 error generated.
```

Reviewers: dblaikie, akyrtzi

Reviewed By: dblaikie

Subscribers: joerg, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D34153

llvm-svn: 305319