[C++20] [Modules] [Reduced BMI] Generate the function body from implicitly...

[C++20] [Modules] [Reduced BMI] Generate the function body from implicitly instantiated class and constant variables

After this patch, we will generate the function body from implicitly
instantiated class. This is important for consumers with same
template arguments. Otherwise the consumers won't see the function body.
Since the consumers won't instantiate the templates again if they find an
instantiation.

Also we will generate the variable definition if the variable is
non-inline but known as constant. Such variables may not affect the
ABI, but they may get involved into the compile time constant computation
in the consumer's code. So we have to generate such definitions.