Strange clang behaviour

Compiling logs

% gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix

% gcc test.c
Undefined symbols for architecture x86_64:
  "_foo", referenced from:
      _main in test-696342.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

% gcc -O2 test.c

IR output (-O0)

; ModuleID = '/var/folders/28/3w_1bq9s3p96j2xy5xmbvxjr0000gn/T/tmpppDdd6/code.c'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.10.0"

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1
  %2 = call i32 (...)* @foo()
  ret i32 %2
}

declare i32 @foo(...) #1

attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)"}

IR output (-O2)

; ModuleID = '/var/folders/28/3w_1bq9s3p96j2xy5xmbvxjr0000gn/T/tmpEzyLyo/code.c'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.10.0"

; Function Attrs: nounwind readnone ssp uwtable
define i32 @main() #0 {
  ret i32 5
}

attributes #0 = { nounwind readnone ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)"}

test.c

inline int foo() {
  return 5;
}

int main() {
  return foo();
}