Since `goog.getMsg` does not process ICUs (post-processing is required via goog.i18n.MessageFormat, https://google.github.io/closure-library/api/goog.i18n.MessageFormat.html) and placeholder format used for ICUs and regular messages inside `goog.getMsg` are different, the current implementation (that assumed the same placeholder format) needs to be updated. This commit updates placeholder format used inside ICUs from `{$placeholder}` to `{PLACEHOLDER}` to better align with Closure. ICU placeholders (that were left as is prior to this commit) are now replaced with actual values in post-processing step (inside `i18nPostprocess`).
PR Close#31459
Prior to this change i18n block bindings were converted to Expressions right away (once we first access them), when in non-i18n cases we processed them differently: the actual conversion happens at instructions generation. Because of this discrepancy, the output for bindings in i18n blocks was generated incorrectly (with invalid indicies in pipeBindN fns and invalid references to non-existent local variables). Now the bindings processing is unified and i18nExp instructions should contain right bind expressions.
PR Close#28969
Andrew Kushnir