[clang] [clang][Sema] Avoid out-of-memory crash on huge designated initializer indices (PR #205503)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

There's a significant difference in the actual memory usage in different scenarios.

- An array that's actually zero-initialized takes no memory until runtime (we stick it in bss).
- An array that has trailing zeros takes up no memory in the compiler, and space proportional to the runtime size of the array in the object file.
- An integer array initialized with #embed uses something like 2-3x the runtime size of the array in the compiler.  We store the initializer in the clang AST, then in LLVM IR, then in the object file.  We can possibly remove one of those copies if we care enough.
- An array with leading zeros costs 8 bytes per implicitly initialized element (one pointer per element in the InitListExpr).
- If we can't constant-evaluate the initializer (`struct A { A(); A(int); };A x[10] = {[9] = 1};`), implicitly initialized elements become very expensive because we don't emit a loop in LLVM IR.

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It's hard to set good thresholds given the differences.  I don't really want to reject `#embed`'ing 1MB files by default.

https://github.com/llvm/llvm-project/pull/205503