Xcode: Fix compiler path detection for iOS tools on Xcode <= 5.0 (#15237)

Since commit v3.1.0-rc1~1^2~1 (Xcode: Fix compiler id detection for iOS
tools, 2014-10-22) our compiler id detection project sets the product
type to 'com.apple.product-type.bundle.unit-test'.  This causes the Ld
command line on which we match the path to the compiler to have a
'CompilerIdC.xctest/' component.  The commit updated our regex to match
this, but placed it before the extra './' component that Xcode 5.0 and
below produce.  Xcode <= 5.0 prints '/./CompilerIdC.xctest/', so switch
the order of the two components in the regex to match it.
Merge topic 'xcode-ios-compiler-id' into next

7f89552a Xcode: Fix compiler path detection for iOS tools on Xcode <= 5.0 (#15237)