Set the "implicitly inline" bit on a method as soon as we see a definition

Set the "implicitly inline" bit on a method as soon as we see a definition
within the class.  Teach IR gen to look for function definitions in record
lexical contexts when deciding whether to emit a function whose address
was taken.  Fixes PR8789.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121833 91177308-0d34-0410-b5e6-96231b3b80d8