Remove SkSL::String + string_view concatenation.

std::string unfortunately does not have a `string + string_view`
plus operator. (https://stackoverflow.com/q/44636549/291737)
This brings SkSL::String one step closer to matching std::string's API.

Change-Id: I44fd4538a9433442d2f34e63ebd120f3377fbb70
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503343
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
21 files changed