Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
And unbreak the LLDB testsuite.
This patch fixes three problems with the original implementation:
- Use SILBuilderWithScope instead of SILBuilder to avoid holes in the
lexical scopes.
- Use an artificial location for stores to the alloca to avoid the debugger
stopping before the variable is initialized.
- Recognize debug_value_addr instructions referring to an alloc_stack
instruction to avoid introducing an extra indirection in the debug info.
rdar://problem/31975108
