[clang] Add clang/docs/FunctionEffectAnalysis.rst. (PR #109855)

[Chris Apple via cfe-commits]

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+Function Effect Analysis
+========================
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+Introduction
+============
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+Clang Function Effect Analysis is a C++ language extension which can warn about "unsafe"
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cjappl wrote:

It is not only C++, but also works in C I think?

https://github.com/llvm/llvm-project/pull/109855