Folly Changes for Clang, revision 1

Folly Changes for Clang, revision 1

Summary:
Depends on D829162 and D830052

A few changes are needed to get folly compiling with clang.
There are likely more changes to compile all of folly

---folly/Bits.cpp
+++folly/folly-config.h
Moved ifunc check, and added an additional check for clang, instead of hard
coding it to be 1

~~~folly/Range.cpp
Moved the declaration of a function to get rid of clang's unused function
warning.

Test Plan:
This bit of code was a dependency in another project.
That code compiled as it needed to

See other diff for more test details

Reviewed By: oyamauchi@fb.com

FB internal diff: D830075