Fix builtin class string instantiation

[HenkPoley]

Fixes #11491

Replaces #11502 with a narrower fix.

Since 3.4.0 @ 99e6cd8 (2019-06-03) Psalm can crash when a string literal containing a built-in/runtime class name is used for dynamic instantiation:

$throwable_class = "\\AssertionError";
throw new $throwable_class("failed");

This PR keeps the fix at the point where the behavior is needed: dynamic instantiation analysis in NewAnalyzer.

Before validating the classlike for new $class_name, Psalm now ensures reflected storage exists when all of these are true:

• the class name is already known to Psalm as an existing classlike,
• storage is not already present,
• PHP already has the class/interface/enum/trait loaded.

The runtime existence checks use *_exists(..., false), so this does not trigger autoloading while analyzing arbitrary values.

Tests

Added coverage in ClassLikeStringTest for instantiating \AssertionError through a string variable with allow_string_standin_for_class is enabled.

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