[lldb][SymbolFileDWARF][NFC] Add FindFunctionDefinition helper (#152733)

Factors out code to locate a definition DIE from a `FunctionCallLabel`
into a helper. This will be useful for an upcoming refactor that extends
`FindFunctionDefinition`.

Drive-by:
* adjusts some error messages in the `FunctionCallLabel` support code.
GitOrigin-RevId: ef30dd34e9ac0bbca9d5b88a5e78b3a4c1167e6c
3 files changed