[CIR] Add constant record ILE support (#155663)
This patch adds basic support for constant record initializer list
expressions. There's a couple of limitations:
* No zero initialized padding bytes in C mode
* No bitfields
* No designated initializer lists
* Record alignments are not calculated, yet
* ILEs of derived records don't work, yet
* The constant attribute is not propagated to the backend, resulting in
non-constants being emitted in the LLVM IR
