swift234

Re-apply r27206: Convert all external function definitions to declarations when compiling with -Onone.

... with disabled test 1_stdlib/Bit.swift for ios.

Most likely the problem of 1_stdlib/Bit.swift (only on armv7) is just uncovered by this change.
Unfortunately I have no possibility to debug the problem on a device. Therefore I filed rdar://problem/20521110




Swift SVN r27274