[clang] [-Wunsafe-buffer-usage] Speed up warning a bit (PR #226605)

[Hans Wennborg via cfe-commits]

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zmodem wrote:

I think you've effectively moved the type check from here to the caller on line 1897 (the other caller in UnsafeBufferUsageCtorAttrGadget::matches already passes a CXXConstructExpr). So this could take a CXXConstructExpr rather than a Stmt now.

Same for StringViewTwoParamConstructorGadget below.

https://github.com/llvm/llvm-project/pull/226605