Merging r367303:

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r367303 | kadircet | 2019-07-30 12:26:51 +0200 (Tue, 30 Jul 2019) | 15 lines

[clangd] Ignore diags from builtin files

Summary:
This fixes a case where we show diagnostics on arbitrary lines, in an
internal codebase.

Open for ideas on unittesting this.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64863
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llvm-svn: 368682