Master-to-next merge - #261

For ease of reviewing, I've checked in the .expected files from the
AST-based guards library. The next commit accepts output for these tests
and adds tests that use getAST rather than the translation layer.

We have external users editing queries with Visual Studio, and it seems
to automatically add very specific files to `.gitignore`. These changes
cause conflicts between unrelated PRs.

This commit adds all of `/.vs` to `.gitignore`, which should hopfully
make Visual Studio stop adding more entries.

Approved by xiemaisi

… files closer.

…ng().

Approved by xiemaisi

Approved by hvitved, xiemaisi, yh-semmle

.gitignore everything under .vs in repo root dir

The IR for the conversion to bool results in a comparison where the left
hand side is not the result of any expression in the AST, so they can't
be usefully converted back to the AST

This is motivated by test performance; IR compilation happens separately
for each test and takes a bit over a minute, so combining these 8 tests
saves about 10 minutes of test running.

Version: Fix C# and JavaScript Eclipse plugins for 1.18.

IR-based guards library

Approved by yh-semmle

Previously, `Instruction.toString()` returned the same string that is used in IR dumps, which requires numbering all instructions and generating a unique string for each instruction. This is too expensive on large snapshots. I've moved the original code into the new `Instruction.getDumpString()`, and made `Instruction.toString()` just return the opcode plus `getAST().toString()`.

C++: Make `Instruction.toString()` less expensive

cf. https://developer.mozilla.org/en-US/docs/Web/API/DOMException/DOMException

Version: Bump to 1.18.1 dev.

JavaScript: Update model of `DOMException`.

Merge rc/1.18 into master.