[6.3] Implement BasicMacroExpansionContext.buildConfiguration

[jmschonfeld]

• Explanation: Adds an implementation of BasicMacroExpansionContext.buildConfiguration so that unit test-provided build configurations get passed through to generic callers in swift-syntax that use the expansion context
• Scope: Only impacts uses of buildConfiguration on a BasicMacroExpansionContext which are typically created for tests
• Issues:
• Original PRs: Implement BasicMacroExpansionContext.buildConfiguration #3191
• Risk: Low - the scope is small and the change size is minimal
• Testing: I was not able to add a unit test for this, but the change is trivial and straightforward so I have high confidence it is correct and does not introduce a regression
• Reviewers: @DougGregor @ahoppen

[jmschonfeld]

@swift-ci please test