[]byte("string") was simplifying to
[]byte{0: 0x73, 1: 0x74, 2: 0x72, 3: 0x69, 4: 0x6e, 5: 0x67},
but that latter form takes up much more memory in the compiler.
Preserve the string form and recognize it to turn global variables
initialized this way into linker-initialized data.

Reduces the compiler memory footprint for a large []byte initialized
this way from approximately 10 kB/B to under 100 B/B.

See also issue 6643.

R=golang-codereviews, r, iant, oleku.konko, dave, gobot, bradfitz
CC=golang-codereviews
https://golang.org/cl/15930045