Decomposition declarator bindings cannot be used in lambda captures

[tbaederr]

Bugzilla Link	50086
Version	trunk
OS	Linux
CC	@zygoloid

Extended Description

Consider the code at https://godbolt.org/z/qa8q75vG6

The first sample (with struct S) is even taken from clang/test/SemaCXX/cxx1z-decomposition.cpp

The problem seems to be that in SemaLambda.cpp, the code requires the LookupResult to return a VarDecl, but it returns a BindingDecl.

[tbaederr]

I guess this comment in SemaExpr.cpp is relevant:

  // FIXME: Support lambda-capture of BindingDecls, once CWG actually
  // decides how that's supposed to work.

I guess this was being done on purpose in 1879f10

[llvmbot]

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