cmd/compile: don't create 2 Sym's and 2 Node's for every string
For every string constant the compiler was creating 2 Sym's and 2
Node's. It would never refer to them again, but would keep them alive
in gostringpkg. This changes the code to just use obj.LSym's instead.
When compiling x/tools/go/types, this yields about a 15% reduction in
the number of calls to newname and a 3% reduction in the total number of
Node objects. Unfortunately I couldn't see any change in compile time,
but reducing memory usage is desirable anyhow.