[Coverage] Disable forced emission for unmapped decls (#15909)
Disable forced SILGen for lazy functions which are not used for code
coverage reporting.
As a drive-by, fix the forced emission logic for functions profiled for
PGO.
This helps address a compile-time issue (r://39332957).
(cherry picked from commit bb567886d8f42c2d5607df7aacaaf1ed3f986ef6)
