[llvm] [NFC] [docs] point users towards clang remark flags (PR #147820)

[Florian Mayer via llvm-commits]

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@@ -57,6 +57,8 @@ Enabling optimization remarks
 There are two modes that are supported for enabling optimization remarks in
 LLVM: through remark diagnostics, or through serialized remarks.
 
+See also the clang flags `-Rpass` and `-fsave-optimization-record`.
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fmayer wrote:

Sure

https://github.com/llvm/llvm-project/pull/147820