Update tsickle to version 0.21.6 which fixes a bug where input source maps which specified filenames differently than the names supplied to tsc didn't get composed with tsc's source maps. Also adds a test that the bug was fixed.
In order for tsickle's new support for input source maps to work, the tsickleCompilerHost must be used for the initial load of source files, since that's when the inline source maps are read and stripped.
Tsickle transforms typescript code, which can change the location of code.
This can cause issues such as runtime stack traces reporting that errors
were raised on the incorrect line in the orginal source. This change replaces
the DecoratorDownlevelCompilerHost and the TsickleCompilerHost with tsickle's
TsickleCompilerHost, which automatically composes tsickle's source maps with
typescript's source maps, so that line numbers are correctly reported at
runtime.
PR Close#14150