Fix a mangling failure on clang-cl C++17

MethodVFTableLocations in MigrosoftVTableContext contains canonicalized
decl. But, it's sometimes asked to lookup for non-canonicalized decl,
and that causes assertion failure, and compilation failure.

Fixes PR37481.

Patch by Taiju Tsuiki!

Differential Revision: https://reviews.llvm.org/D46929

llvm-svn: 332639