[lldb] DWARFDIE: Follow DW_AT_specification when computing CompilerCo… (#77157)

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Following the specification chain seems to be clearly the expected
behavior of GetDeclContext(). Otherwise C++ methods have an empty
CompilerContext instead of being nested in their struct/class.

Theprimary motivation for this functionality is the Swift plugin. In
order to test the change I added a proof-of-concept implementation of a
Module::FindFunction() variant that takes a CompilerContext, expesed via
lldb-test.

rdar://120553412