[CGDebugInfo] Fix -femit-debug-entry-values crash on os_log_helpers

An os_log_helper FunctionDecl may not have a body. Ignore these for the
purposes of debug entry value emission.

Fixes an assertion failure seen in a stage2 build of clang:

Assertion failed: (FD->hasBody() && "Functions must have body here"),
function analyzeParametersModification

llvm-svn: 365716