Fix clang-tidy warnings in libbcc.

These are proposed changes, looking for upstream changes.
* Add explicit keyword to conversion constructors.
Bug: 28341362
* Use const reference type for read-only parameters.
Bug: 30407689
* Use const reference type for loop index variables to avoid unnecessary copy.
Bug: 30413223
* Use faster overloaded string find function.
Bug: 30411878
Test: build with WITH_TIDY=1

Change-Id: I2a083651ba7aa7cb1b88fb84738d7caf37b4f511
20 files changed