[CIR] New assembly format for function type return

[dkolsen-pgi]

Change the assembly format for cir::FuncType from

!cir.func<!returnType (!argType)>

to

!cir.func<(!argType) -> !returnType>

This change (1) is easier to parse because it doesn't require lookahead, (2) is consistent with the format of ClangIR FuncOp assembly, and (3) is consistent with function types in other MLIR dialects.

Change all the tests to use or to expect the new format for function types.

The contents and the semantics of cir::FuncType are unchanged. Only the assembly format is being changed. Functions that return void in C or C++ are still represented in MLIR as having no return type.

Most of the changes are in parseFuncType and printFuncType and the functions they call in CIRTypes.cpp.

A FuncType::verify function was added to check that an explicit return type is not cir::VoidType.

FuncType::isVoid() was renamed to FuncType::hasVoidReturn()

Some comments for FuncType were improved.

An llvm_unreachable was added to StructType::getKindAsStr to suppress a compiler warning and to catch a memory error that corrupts the RecordKind field. (This was a drive-by fix and has nothing to do with the rest of this change.)

[dkolsen-pgi]

I considered renaming FuncType::getReturnType and FuncType::getReturnTypes so that the difference between them was more clear. But I struggled to find the right names. So instead I just made the documentation for those functions more clear.