internal/lsp: don't store diagnostics for builtin.go
internal/lsp: don't store diagnostics for builtin.go

Fixes golang/go#44866

Change-Id: I9606df932c1b35d91816306fbcb48edc5b023b30
GitHub-Last-Rev: e6f525ed46a08ac77eac3af295c2be684405e88b
GitHub-Pull-Request: golang/tools#364
Reviewed-on: https://go-review.googlesource.com/c/tools/+/386219
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Trust: Nooras Saba‎ <882f285c799705090ca85529b929bbb8eeaf9f87@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools