Handle generic types as parameters of attributes in MetadataDecoder

[jjonescz]

Fixes #66370.
Fixes #55190.

PEAttributeData has the attribute symbol, so this PR changes MetadataDecoder to use attribute argument types from that symbol (instead of reading the metadata directly), hence all cases are handled - including the previously missing generic type parameters.

PIA attribute validation has also been refactored. Previously, it decoded attributes in all cases, now it does that only for SourceAssemblySymbols; whereas it consults metadata directly for PEAssemblySymbols. See #70151 (comment).

[AlekseyTs]

Done with review pass (commit 1)

[AlekseyTs]

                if (signatureHeader.IsGeneric && sigReader.ReadCompressedInteger() != 0)

We might want to have an equivalent check in the method above, just in case. #Closed

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Refers to: src/Compilers/Core/Portable/MetadataReader/MetadataDecoder.cs:1795 in 6c27299. [](commit_id = 6c27299, deletion_comment = False)

[AlekseyTs]

                if (returnTypeCode != SignatureTypeCode.Void)

And this check as well #Closed

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Refers to: src/Compilers/Core/Portable/MetadataReader/MetadataDecoder.cs:1805 in 6c27299. [](commit_id = 6c27299, deletion_comment = False)

[AlekseyTs]

Done with review pass (commit 4), tests are not looked at.

[AlekseyTs]

Done with review pass (commit 23)

[AlekseyTs]

LGTM (commit 25)

[AlekseyTs]

@dotnet/roslyn-compiler For the second review

[AlekseyTs]

@dotnet/roslyn-compiler For the second review

[jaredpar]

@333fred PTAL

[AlekseyTs]

@333fred, @dotnet/roslyn-compiler For the second review

[333fred]

Generally looks good, but I have a question on the structure of the new methods.