[CIR] Upstream ComplexType support

[AmrDeveloper]

Overview

Upstream, the basic support for Complex types. The initial change should add support for declaring and initialising global variables that are a Complex of existing scalar types. Follow-up patches can add support for local variables and operations.

Suggested minimal test case

int _Complex ci1;
int _Complex ci2 = {1, 2};

double _Complex cd1
double _Complex cd2 = {1.0, 2.0};

Existing incubator tests:

• clang/test/CIR/CodeGen/complex.c
• clang/test/CIR/CodeGen/complex-cast.c
• clang/test/CIR/CodeGen/complex-arithmetic.c