[CIR][CodeGen] Support trailing_zeros for constant string literals

[ivanmurashko]

The patch resolves issue #248. It can be considered a subsequent patch to #373, where the case of empty strings was processed.

The new patch adds processing for non-empty strings that may contain trailing zeros, such as:

char big_string[100000] = "123";

That is converted to

@big_string = #cir.const_array<"123" : !cir.array<!s8i x 3>, trailing_zeros> : !cir.array<!s8i x 100000>

It's worth noting that ordinary strings that are terminated with a single zero also get the trailing zeros, for instance

char string[] = "whatnow";

Is converted to

@string = #cir.const_array<"whatnow" : !cir.array<!s8i x 7>, trailing_zeros> : !cir.array<!s8i x 8>

[ivanmurashko]

@bcardosolopes, could you look at the patch?

[bcardosolopes]

Awesome, few remaining things:

• Add a new testcase to test this propagates all the way down to LLVM - see CodeGen/abstract-cond.c and similar for RUN line examples.
• This is a good opportunity for going the extra length and getting rid of ZeroInitConstOp, but it's fine if you do that as part of another PR.

[ivanmurashko]

@bcardosolopes could you look at the test I created?

I used the following command to created the test data for LLVM-IR

bin/clang -cc1 -triple aarch64-none-linux-android21 -emit-llvm  ../clang/test/CIR/CodeGen/string-literals.c -o /tmp/test.ll

I.e. the check data were created without Clang-IR involvement

[bcardosolopes]

LGTM!

btw, created more issues to track other follow ups, if you are interested feel free to grab them!