[IR] Allow MDString in operand bundles (#110805)
This change implements support of metadata strings in operand bundle
values. It makes possible calls like:
call void @some_func(i32 %x) [ "foo"(i32 42, metadata !"abc") ]
It requires some extension of the bitcode serialization. As SSA values
and metadata are stored in different tables, there must be a way to
distinguish them during deserialization. It is implemented by putting a
special marker before the metadata index. The marker cannot be treated
as a reference to any SSA value, so it unambiguously identifies
metadata. It allows extending the bitcode serialization without breaking
compatibility.
Metadata as operand bundle values are intended to be used in
floating-point function calls. They would represent the same information
as now is passed by the constrained intrinsic arguments.
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