[CIR][CodeGen] Emit cir.resume for synthetic TryOp's in place

[bruteforceboy]

Currently, the following code snippet fails during CIR codegen with exceptions enabled:

#include <string>

void foo(const char *path) {
  std::string str = path;
  str = path;
  str = path;
}

using bin/clang++ tmp.cpp -fclangir -Xclang -emit-cir -S, the error:

error: empty block: expect at least a terminator

Relevant part of the CIR before verification looks like:

%118 = "cir.load"(%114) : (!cir.ptr<!cir.ptr<!cir.int<s, 8>>>) -> !cir.ptr<!cir.int<s, 8>>
"cir.try"() <{catch_types = [#cir.unwind], cleanup, synthetic}> ({
    %123 = "cir.call"(%115, %118) <{ast = #cir.call.expr.ast, callee = @_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEaSEPKc, calling_conv = 1 : i32, exception, extra_attrs = #cir<extra({})>, side_effect = 1 : i32}> ({
    "cir.call"(%115) <{callee = @_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEED1Ev, calling_conv = 1 : i32, extra_attrs = #cir<extra({nothrow = #cir.nothrow})>, side_effect = 1 : i32}> ({
    }) : (!cir.ptr<!cir.struct<class "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>" {!cir.struct<struct "std::__cxx11::basic_string<char>::_Alloc_hider" {!cir.ptr<!cir.int<s, 8>>} #cir.record.decl.ast>, !cir.int<u, 64>, !cir.struct<union "anon.0" padded {!cir.array<!cir.int<s, 8> x 16>, !cir.int<u, 64>, !cir.array<!cir.int<u, 8> x 8>} #cir.record.decl.ast>} #cir.record.decl.ast>>) -> ()
    "cir.yield"() : () -> ()
    }) : (!cir.ptr<!cir.struct<class "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>" {!cir.struct<struct "std::__cxx11::basic_string<char>::_Alloc_hider" {!cir.ptr<!cir.int<s, 8>>} #cir.record.decl.ast>, !cir.int<u, 64>, !cir.struct<union "anon.0" padded {!cir.array<!cir.int<s, 8> x 16>, !cir.int<u, 64>, !cir.array<!cir.int<u, 8> x 8>} #cir.record.decl.ast>} #cir.record.decl.ast>>, !cir.ptr<!cir.int<s, 8>>) -> !cir.ptr<!cir.struct<class "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>" {!cir.struct<struct "std::__cxx11::basic_string<char>::_Alloc_hider" {!cir.ptr<!cir.int<s, 8>>} #cir.record.decl.ast>, !cir.int<u, 64>, !cir.struct<union "anon.0" padded {!cir.array<!cir.int<s, 8> x 16>, !cir.int<u, 64>, !cir.array<!cir.int<u, 8> x 8>} #cir.record.decl.ast>} #cir.record.decl.ast>>
    "cir.store"(%123, %116) : (!cir.ptr<!cir.struct<class "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>" {!cir.struct<struct "std::__cxx11::basic_string<char>::_Alloc_hider" {!cir.ptr<!cir.int<s, 8>>} #cir.record.decl.ast>, !cir.int<u, 64>, !cir.struct<union "anon.0" padded {!cir.array<!cir.int<s, 8> x 16>, !cir.int<u, 64>, !cir.array<!cir.int<u, 8> x 8>} #cir.record.decl.ast>} #cir.record.decl.ast>>, !cir.ptr<!cir.ptr<!cir.struct<class "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>" {!cir.struct<struct "std::__cxx11::basic_string<char>::_Alloc_hider" {!cir.ptr<!cir.int<s, 8>>} #cir.record.decl.ast>, !cir.int<u, 64>, !cir.struct<union "anon.0" padded {!cir.array<!cir.int<s, 8> x 16>, !cir.int<u, 64>, !cir.array<!cir.int<u, 8> x 8>} #cir.record.decl.ast>} #cir.record.decl.ast>>>) -> ()
    "cir.yield"() : () -> ()
}, {
^bb0:              <--- EMPTY BLOCK
}) : () -> ()

There is an empty block! If you extend the snippet with more str = path;, you get more empty blocks...

The issue is the cir.resume ops which should be in those empty blocks from synthetic TryOp's aren't linked properly during the cleanup.

My suggestion: We should explicitly add cir.resume for synthetic tryOp's, because we already know they have just an unwind handler. So, during CIRGenCleanup we don't need to add cir.resume for synthetic TryOp's. This PR adds this and a test.

[bcardosolopes]

Awesome, elegant fix! One minor nit below