Update upload-symbols with better logging and reading DEVELOPER_DIR to get Xcode path for CoreSymbolication

[tejasd]

Hey there! So you want to contribute to a Firebase SDK?
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Discussion

This is based on feedback in #15896.

Testing

• Verified w/ integration test
• Verified locally

API Changes

This change reads environment variable DEVELOPER_DIR in upload-symbols for CoreSymbolication.

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[tejasd]

Instructions for reviewing this (since it's a binary):

• ./upload-symbols --version
• codesign -dv --verbose=2 upload-symbols

[ncooke3]

LGTM for changelog