The return type of getNumRuntimeFunctionCounters is defined as uint64_t in RuntimeInvocationsTracking.cpp, but it has return type Int in RuntimeFunctionCounters.swift.
Found when compiling the stdlib for WebAssembly, as WebAssembly validates return types. uint64_t corresponds to i64, but Int is i32, so the program fails validation.
Also update how the variable is managed in the build system to allow the test to be conditional based on it, and make it more natural to set it on the command line.
Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
