Change __extension__ to disable only diagnostics controlled by -pedantic.

Change __extension__ to disable only diagnostics controlled by -pedantic.

This changes clang to match GCC's behavior for __extension__, which temporarily
disables the -pedantic flag.  Warnings that are enabled without -pedantic
are not affected.  Besides the general goodness of matching GCC's precedent,
my motivation for this is that macros in the arm_neon.h header need to use
__extension__ to avoid pedantic complaints about their use of statement
expressions, yet we still want to warn about incompatible pointer arguments
for those macros.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141804 91177308-0d34-0410-b5e6-96231b3b80d8