Allow -profile-guided-section-prefix more than once

[marksantaniello]

Summary

At present, -profile-guided-section-prefix is a cl::Optional option, which means it demands to be passed exactly zero or one times. Our build system makes it pretty tricky to guarantee this. We often accidentally pass the flag more than once (but always with the same "false" value) which results in an error, after which compilation fails:

clang (LLVM option parsing): for the -profile-guided-section-prefix option: may only occur zero or one times!

While we work on improving our build system, it also seems reasonable just to allow -profile-guided-section-prefix to be passed more than once, by to cl::ZeroOrMore. Quoting the documentation:

The cl::ZeroOrMore modifier ... indicates that your program will allow the option to be specified zero or more times.
...
If an option is specified multiple times for an option of the cl::opt class, only the last value will be retained.

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

static cl::opt<bool> StressExtLdPromotion(
     cl::desc("Stress test ext(promotable(ld)) -> promoted(ext(ld)) "
              "optimization in CodeGenPrepare"));
 
 static cl::opt<bool> DisablePreheaderProtect(
     "disable-preheader-prot", cl::Hidden, cl::init(false),
     cl::desc("Disable protection against removing loop preheaders"));
 
 static cl::opt<bool> ProfileGuidedSectionPrefix(
-    "profile-guided-section-prefix", cl::Hidden, cl::init(true),
+    "profile-guided-section-prefix", cl::Hidden, cl::init(true), cl::ZeroOrMore,
     cl::desc("Use profile info to add section prefix for hot/cold functions"));
 
 static cl::opt<unsigned> FreqRatioToSkipMerge(
     "cgp-freq-ratio-to-skip-merge", cl::Hidden, cl::init(2),
     cl::desc("Skip merging empty blocks if (frequency of empty block) / "
              "(frequency of destination block) is greater than this ratio"));
 
 static cl::opt<bool> ForceSplitStore(