[clang] [clang][modules] Allow module maps with textual headers to be non-affecting (PR #89441)

[Jan Svoboda via cfe-commits]

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+// This test checks that a module map with a textual header can be marked as
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jansvoboda11 wrote:

(And thanks for the extended test case!)

https://github.com/llvm/llvm-project/pull/89441