[Parse] Use empty RecoveryExpr when if/while/do/switch conditions fail to parse

This allows the body to be parsed.
An special-case that would replace a missing if condition with OpaqueValueExpr
was removed as it's now redundant (unless recovery-expr is disabled).

For loops are not handled at this point, as the parsing is more complicated.

Differential Revision: https://reviews.llvm.org/D113752