[lldb] Let callers of GetSharedModule skip symbol locating (NFC) (#214829)
GetSharedModule falls back on the symbol locator plugins when the spec
does not already name a binary it can open, and it does so while holding
the global module list lock. Locating a binary or symbol file can
potentially be quite slow, so we should have the option to opt out of
that, for example, if we already did the search upfront.