[clangd] Fix unintentionally loose fuzzy matching, and the tests masking it.

[clangd] Fix unintentionally loose fuzzy matching, and the tests masking it.

Summary:
The intent was that [ar] doesn't match "FooBar"; the first character must match
a Head character (hard requirement, not just a low score).
This matches VSCode, and was "tested" but the tests were defective.

The tests expected matches("FooBar") to fail for lack of a match. But instead
it fails because the string should be annotated - matches("FooB[ar]").
This patch makes matches("FooBar") ignore annotations, as was intended.

Fixing the code to reject weak matches for the first char causes problems:
-  [bre] no longer matches "HTMLBRElement".
   We allow matching against an uppercase char even if we don't think it's head.
   Only do this if there's at least one lowercase, to avoid triggering on MACROS
-  [print] no longer matches "sprintf".
   This is hard to fix without false positives (e.g. [int] vs "sprintf"])
   This patch leaves this case broken. A future patch will add a dictionary
   providing custom segmentation to common names from the standard library.

Fixed a couple of index tests that indirectly relied on broken fuzzy matching.
Added const in a couple of missing places for consistency with new code.

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D44003

llvm-svn: 326721