SymbolFileDWARF: Unconditionally scan through clang modules. NFCish

When looking up a type by name, also scan through any referenced Clang
modules regardsless of whether a type with this name has been
found. This is NFCish (= a potential performance regression) for Clang
projects, but necessary in mixed Swift and Objective-C projects (and
tested in swift-lldb).

This only affects projects compiled with -gmodules that were not run
through dsymutil.

llvm-svn: 368345