[clangd] Use resolveTypeToRecordDecl() to resolve the type of a base specifier...

[clangd] Use resolveTypeToRecordDecl() to resolve the type of a base specifier during heuristic resolution

The code for resolving the type of a base specifier was inside
CXXRecordDecl::lookupDependentName(), so this patch reimplements
lookupDependentName() in HeuristicResolver.

Fixes https://github.com/clangd/clangd/issues/1657

Differential Revision: https://reviews.llvm.org/D153248