[clang] [clang] MangledSymbol: remove pointless copy of vector (PR #90012)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@shafik I wonder how checking size of a vector is now considered an expensive check.
(I stumbled upon it while reviewing)
https://github.com/llvm/llvm-project/blob/47682e4b4a0c8e7637d65868a7208aa6806a50f4/clang/lib/Frontend/InterfaceStubFunctionsConsumer.cpp#L99-L101

https://github.com/llvm/llvm-project/pull/90012