[DebugInfo] Add DW_ATE_complex_float case to assert in isUnsignedDIType

Without this patch a `DW_ATE_complex_float` encoding trips an assertion in
`DebugHandlerBase::isUnsignedDIType` with the message `"Unsupported
encoding"`.

By adding a case to the `assert` for `DW_ATE_complex_float` it becomes
supported, behaving in the same way as the already supported `DW_ATE_float`
type (return false).

Note: For the reported reproducer:

    #include <complex.h>
    int main() {
      long double complex r1;
    }

The assertion isn't tripped without assignment tracking because instcombine
deletes everything, including the `dbg.declare`, without recovering any
location information. Whereas with assignment tracking we track a zeroing
memset that is emitted by clang.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D151795