runtime/pprof,cmd/compile: simplified symbol names for generics break PGO matching

[prattmic]

For functab, and thus stack traces and symbol names in pprof profile outputs, the compiler simplifies symbol names like foo[go.shape.int64] to just foo[...].

When reading a profile for PGO, the compiler will attempt to match symbols and fail because the profile symbol name does not match the actual symbol name.

This could be addressed from either end:

• The profile could contain the full symbol name (Function.name = "foo[...]", Function.system_name = "foo[go.shape.int64]" perhaps?).
• Or, the compiler could match foo[...] to any instantiation of foo.

cc @cherrymui @aclements

[cherrymui]

I'd prefer the first option. For the same generic function, some instantiation may be hot and some other may be cold. So it would be good to distinguish them. Especially if we want to do PGO-based specification/stenciling later.

[cherrymui]

I'll look into it.