[clangd] Make completion scores use 0-1 floats internally.

Summary:
This scale is much easier to mix with other signals, such as fuzzy match strength.
Mostly NFC, but it does reorder some low-priority items that get folded together at a score of 0 (see completion-qualifiers.test).
Removed the exact sortText from the testcases, because it's the ranking that we want to test.

Reviewers: hokein

Subscribers: ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D40089

llvm-svn: 318927