[lldb] Delete IRExecutionUnit::SearchSpec

IRExecutionUnit::SearchSpec is a struct that encapsulates information
needed to look for a symbol. Specifically, it is comprised of a name
represented with a ConstString and a FunctionNameType mask.
Because the mask is unused (effectively always set to
eFunctionNameTypeFull), we can remove the mask and replace all uses with
eFunctionNameTypeFull.  After doing that, SearchSpec is effectively a
wrapper around a ConstString.

As an aside, SearchSpec is similar in purpose to Module::LookupInfo. I
briefly considered replacing uses of SearchSpec with LookupInfo, but
the current code only cares about symbol names (treating them as
eFunctionNameTypeFull). This code does care about language type, so
LookupInfo may be appropriate for IRExecutionUnit in the future.

Differential Revision: https://reviews.llvm.org/D109384