[lldb] Silence narrowing conversion warning with MSVC

Fixes:
```
[13/270] Building CXX object tools\lldb\unittests\Core\CMakeFiles\LLDBCoreTests.dir\DumpDataExtractorTest.cpp.obj
C:\git\llvm-project\lldb\unittests\Core\DumpDataExtractorTest.cpp(140): warning C4305: 'argument': truncation from 'double' to 'const std::complex<float>::_Ty'
```