Quadratic scaling in processing of designated initializers

[amonakov]

printf 'x(%d)\n' $(seq 10000) > x10000.h

struct S {
#define x(n) int m##n;
#include "x10000.h"
#undef x
} s = {
#define x(n) .m##n = n,
#include "x10000.h"
#undef x
};

needs more than a second to compile, and scales quadratically with the number of fields. perf report shows 42% in CheckDesignatedInitializer, 34% in CheckStructUnionTypes and 17.5% in FieldDecl::getCanonicalDecl.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Alexander Monakov (amonakov)

```sh printf 'x(%d)\n' $(seq 10000) > x10000.h ``` ```c struct S { #define x(n) int m##n; #include "x10000.h" #undef x } s = { #define x(n) .m##n = n, #include "x10000.h" #undef x }; ``` needs more than a second to compile, and scales quadratically with the number of fields. `perf report` shows 42% in CheckDesignatedInitializer, 34% in CheckStructUnionTypes and 17.5% in FieldDecl::getCanonicalDecl.