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The code to rebase the path to the sdk was duplicated between clang
and swift. Use a local variable initialised with the value that was
computed in the two location, then use it to configure "-isysroot"
for clang and "-sdk" for swiftc.

Bug: none
Change-Id: Ic3794ceeea84c67404763cf5d86fc32c201e3a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471357
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817444}