Huge slowdown introduced in r91449

Tracking a slowdown observed after a clang update, I've bisected the
problem to commit r91449.

$ ~/llvm_opt/Release/bin/clang -w -fsyntax-only gcc.c

real 0m3.683s
user 0m3.354s
sys 0m0.325s

$ time ~/llvm_opt/Release/bin/clang -w -fsyntax-only gcc.c

real 0m38.764s
user 0m38.378s
sys 0m0.374s

The very large source file gcc.c (22 MB) is from:

Please file a bug report for this, thanks!

- Daniel