SwiftCompilerSources/SIL: Fix APInt assertion failure on rebranch
The assertion is hit through `TypeValueInst.simplify` when constructing
an integer literal instruction with a negative 64-bit `Swift.Int` and a
bit width of 32 (the target pointer bit width for arm64_32 watchOS).
This happens because we tell the `llvm::APInt` constructor to treat the
input integer as unsigned by default in `getAPInt`, and a negative
64-bit signed integer does not fit into 32 bits when interpreted as
unsigned.
Fix this by flipping the default signedness assumption for the Swift API
and introducing a convenience method for constructing a 1-bit integer
literal instruction, where the correct signedness assumption depends on
whether you want to use 1 or -1 for 'true'.
In the context of using an integer to construct an `llvm::APInt`, there
are 2 other cases where signedness matters that come to mind:
1. A non-decimal integer literal narrower than 64 bits, such as
`0xABCD`, is used.
2. The desired bit width is >64, since `llvm::APInt` can either
zero-extend or sign-extend the 64-bit integer it accepts.
Neither of these appear to be exercised in SwiftCompilerSources, and
if we ever do, the caller should be responsible for either (1)
appropriately extending the literal manually, e.g.
`Int(Int16(bitPattern: 0xABCD))`, or (2) passing along the appropriate
signedness.
